# Mythbusters 6/12/2013 - Hypermiling and Crash Cushion



## GoPackGo (Dec 29, 2012)

Something about the synthetic cadaver was absolutely terrifying.

Of course Adam had an old Volvo!

I found it really annoying how they danced around ever saying that the police officers were fat. Plus sized. Big. Large. Wide. Heavyset. Just say fat!


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## john4200 (Nov 1, 2009)

They said they have used 44 Buster variations over the years!

Did they mention the title of the movie that had the guy between the two fat cops in the car accident? I remember the scene, but I cannot remember the rest of the movie (or the name).


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## Kamakzie (Jan 8, 2004)

john4200 said:


> They said they have used 44 Buster variations over the years!
> 
> Did they mention the title of the movie that had the guy between the two fat cops in the car accident? I remember the scene, but I cannot remember the rest of the movie (or the name).


I think they said the movie was called Headhunters?


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## Church AV Guy (Jan 19, 2005)

I thin you can extend your gas mileage by driving at 45MPH on the freeway, but at the expense of vastly shortening your personal lifespan.

That was quite the impact when the truck hit the car. I too thought the cops would provide cushion.


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## secondclaw (Oct 3, 2004)

GoPackGo said:


> Something about the synthetic cadaver was absolutely terrifying.
> 
> Of course Adam had an old Volvo!
> 
> I found it really annoying how they danced around ever saying that the police officers were fat. Plus sized. Big. Large. Wide. Heavyset. Just say fat!


Jamie did say fat before the last test.


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## GoPackGo (Dec 29, 2012)

secondclaw said:


> Jamie did say fat before the last test.


You're right. I wrote my statement before my viewing of the episode had ended.


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## john4200 (Nov 1, 2009)

Kamakzie said:


> I think they said the movie was called Headhunters?


Thanks, that is it. Now I remember.


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## JMikeD (Jun 10, 2002)

Church AV Guy said:


> I thin you can extend your gas mileage by driving at 45MPH on the freeway


Depends on the car. The "sweet spot" for economy seems to be in the 40-60 mph (according to auto websites), but depends on gearing, aerodynamics, weight, etc. In general, smaller, lighter, more aerodynamic cars will get their best mileage at the higher end of that range. Bigger, heavier, less aerodynamic vehicles will get their best mileage at lower speeds.


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## gschrock (Dec 28, 2001)

And yet, the best gas mileage I ever got on my old grand cherokee was travelling across south dakota, where I got about 22 mpg, and the speed limit there at the time was around 75. (I did a lot of expressway driving on that trip, and even in other states, like I think it was iowa where it was a brutal 55, always did less on gas mileage.)


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## john4200 (Nov 1, 2009)

gschrock said:


> And yet, the best gas mileage I ever got on my old grand cherokee was travelling across south dakota, where I got about 22 mpg, and the speed limit there at the time was around 75. (I did a lot of expressway driving on that trip, and even in other states, like I think it was iowa where it was a brutal 55, always did less on gas mileage.)


If everything else were equal, there is zero chance that a Grand Cherokee would get better gas mileage at 75mph than 55mph. The air resistance at 75mph is much higher than at 55mph, especially for a vehicle shaped like a GC. Any engine efficiency vs. rpm factors are going to be swamped by the much higher air resistance at 75mph.

So, to explain your findings you have to look at other factors. One would be whether you measured the gas mileage accurately and whether you remember correctly. Another important factor is headwinds and tailwinds. Traffic (varying speeds). Hills and mountains. Tire pressure. Whether windows were open. Air conditioning. Load. And others.


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## vman41 (Jun 18, 2002)

JMikeD said:


> Depends on the car. The "sweet spot" for economy seems to be in the 40-60 mph (according to auto websites), but depends on gearing, aerodynamics, weight, etc. In general, smaller, lighter, more aerodynamic cars will get their best mileage at the higher end of that range. Bigger, heavier, less aerodynamic vehicles will get their best mileage at lower speeds.


In my experience, 45 MPH is where transmissions generally shift into overdrive, so getting best mileage there is not surprise. My current car is 5 years old and just missed the transition to transmissions with more gears, so there might be a sweet spot now somewhat higher up.


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## scooterboy (Mar 27, 2001)

So what were their (general) findings on hypermiling?


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## john4200 (Nov 1, 2009)

scooterboy said:


> So what were their (general) findings on hypermiling?


You can increase your mileage by following several techniques that they tested, but it is difficult, and you cannot double your mileage. Also, although not stated, it was seen rather clearly that it may not be worth following several of the techniques unless you are really desperate to increase your mileage: windows up and A/C off was brutal; 45mph on the highway was causing traffic jams and getting other drivers angry; turning off the engine at a red light often resulted in very slow off the mark and honks from other drivers; avoiding braking on downhill runs made the passenger cringe when it felt like the car was nearly out of control.


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## secondclaw (Oct 3, 2004)

I've done hypermiling when running out of gas, and thought their finding on A/C versus open windows from an earlier episode was pretty interesting. I was actually hoping they would do another HM thing - accelerating 5mph above posted speed limit then coasting down ... I've always been curious if that method actually does anything but been too lazy to try it myself. I suppose they don't want to break the law on TV, even if its a 5mph over limit.


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## RonDawg (Jan 12, 2006)

john4200 said:


> You can increase your mileage by following several techniques that they tested, but it is difficult, and you cannot double your mileage. Also, although not stated, it was seen rather clearly that it may not be worth following several of the techniques unless you are really desperate to increase your mileage: windows up and A/C off was brutal; 45mph on the highway was causing traffic jams and getting other drivers angry; turning off the engine at a red light often resulted in very slow off the mark and honks from other drivers; avoiding braking on downhill runs made the passenger cringe when it felt like the car was nearly out of control.


They also said these methods were legal and going 45 on a posted 65 freeway (as many of those stretches were) is at the very edge of legality, if not safety. I would never recommend anybody drive at that speed on a California freeway unless conditions are such that you really shouldn't go any faster, such as during a rainstorm, and I certainly wouldn't go into the left lane like Tory did in one shot.

Turning off the engine at traffic lights has already been proven and has been incorporated into auto stop-start systems. When I was in Europe last year, I rented a BMW with auto stop-start and I didn't find it to be a problem at all. The car started right up with almost no delay once I took my foot off the brake.

They could have also controlled their speed on the downhill stretches by simply downshifting the transmission and taking advantage of engine braking.


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## Waldorf (Oct 4, 2002)

RonDawg said:


> Turning off the engine at traffic lights has already been proven and has been incorporated into auto stop-start systems. When I was in Europe last year, I rented a BMW with auto stop-start and I didn't find it to be a problem at all. The car started right up with almost no delay once I took my foot off the brake.


I'm currently driving a Hyundai Sonata Hybrid rental and find the engine cut-off at stops to be a bit unnerving. Starts right up like a golf cart, though.


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## vertigo235 (Oct 27, 2000)

I assume you can turn it off. What about those times when you're in heavy traffic and Need the air conditioner while you're stopped?


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## john4200 (Nov 1, 2009)

RonDawg said:


> They could have also controlled their speed on the downhill stretches by simply downshifting the transmission and taking advantage of engine braking.


No, you missed the point. They were avoiding braking of any kind in order to keep their momentum up and save fuel. If you brake (with the brakes, or the engine) before going into a turn, then you need to use more fuel later to speed up again.


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## Hank (May 31, 2000)

They prob could have shifted into neutral on the down slopes and idle the engine, but I know that's not generally legal.


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## brianric (Aug 29, 2002)

vertigo235 said:


> I assume you can turn it off. What about those times when you're in heavy traffic and Need the air conditioner while you're stopped?


The 2012 Camry Hybrid has electronic air conditioning and water pump.


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## JYoung (Jan 16, 2002)

RonDawg said:


> They also said these methods were legal and going 45 on a posted 65 freeway (as many of those stretches were) is at the very edge of legality, if not safety. I would never recommend anybody drive at that speed on a California freeway unless conditions are such that you really shouldn't go any faster, such as during a rainstorm, and I certainly wouldn't go into the left lane like Tory did in one shot.


I'm not sure but I thought that I saw a CHP escort for Kari and Grant when they ran out of gas on the hypermile lap.
Perhaps that prevented them from being victims of road rage.


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## jeff92k7 (Jan 18, 2006)

JYoung said:


> I'm not sure but I thought that I saw a CHP escort for Kari and Grant when they ran out of gas on the hypermile lap.
> Perhaps that prevented them from being victims of road rage.


If you look closely, you can see Police escorts and traffic control in all their "on the road" segments. They do a pretty good job of editing it out, but you can still catch glimpses of them. Anything they have done on the public roads in the last few years has had a police presence (Blocking traffic, escort, shutting down roads, etc.)


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## gschrock (Dec 28, 2001)

jeff92k7 said:


> If you look closely, you can see Police escorts and traffic control in all their "on the road" segments. They do a pretty good job of editing it out, but you can still catch glimpses of them. Anything they have done on the public roads in the last few years has had a police presence (Blocking traffic, escort, shutting down roads, etc.)


There was an episode I think last season where they mentioned how they have to have police escort for anything they do on the roads.


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## aaronw (Apr 13, 2001)

Generally speaking, any car will be the most fuel efficient per mile in the topmost gear, at the lowest RPMs that will keep it from stalling in that gear. Fuel economy is directly tied to engine RPMs, and how 'much' those engine RPMs push you through the air. So, the more gears, the faster the 'most efficient' speed is, except that you run into air resistance at a point that may push you down a gear or two.

As far as avoiding braking, sure. You want to keep your KE as high as possible, and if you have a lot of PE at the top of a hill, then at the bottom you'll have a lot of KE. Probably another tip for hypermiling is accelerate just enough (when possible) to coast over a hill when climbing it and then get all that KE back going down. As others point out, if you go over the top of a hill with a lot of KE, you may end up going quite fast and have to 'throw away' some of that energy (either via downshifting or breaking) on the way down. Electric cars with regenerative brakes can put some of that lost KE back into the system, whereas traditional brakes just turn it into heat.


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## midas (Jun 1, 2000)

I don't see how the long road test proved anything. As a simple, but extreme, example, lets say the first 50 miles was mainly flat highway driving. The first run, they get exactly 50 miles out of their fuel allocation. The second run they start the hypermiling. They get past their 50 mile mark, but at the 51 mile mark the road turns into pure uphill and for some reason it's bumper to bumper. They might get 5 extra miles before running out of fuel. 

So,did they only get 10% better mileage? Of course not. It could have been double. It could have been triple. The only way to tell is under identical conditions for the next 50 miles as they had on the first 50 miles. 

And with real traffic, you couldn't even guarantee that the first 50 miles of the trip were done under identical conditions. Either run could have had more traffic or different wind conditions. 

Finally, while they tried to follow hypermiling theory, none of them were well practiced experts. What they should have done was have one of those lunatic hypermiling nuts run the second leg at the same time the MB gang ran the first leg. And compare the fuel consumed at a measured distance rather than how much further they went. 

As always, this show is more about fun than true scientific proff.


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## pteronaut (Dec 26, 2009)

RonDawg said:


> They also said these methods were legal and going 45 on a posted 65 freeway (as many of those stretches were) is at the very edge of legality, if not safety. I would never recommend anybody drive at that speed on a California freeway unless conditions are such that you really shouldn't go any faster, such as during a rainstorm, and I certainly wouldn't go into the left lane like Tory did in one shot.
> 
> Turning off the engine at traffic lights has already been proven and has been incorporated into auto stop-start systems. When I was in Europe last year, I rented a BMW with auto stop-start and I didn't find it to be a problem at all. The car started right up with almost no delay once I took my foot off the brake.
> 
> They could have also controlled their speed on the downhill stretches by simply downshifting the transmission and taking advantage of engine braking.


Volkswagen had dabbled with auto-stop/start systems for over 30 years, the 1981 Derby (EU Polo saloon) came with such a system in it's 1.1L 'Formel E' model.


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## cwerdna (Feb 22, 2001)

midas said:


> Finally, while they tried to follow hypermiling theory, none of them were well practiced experts.


Yep. I may be smeeking but the ep didn't really show how well they were hypermiling. For example, if they were approaching a red light, they should've shut off their engine at some point before the red light and just coasted to the intersection.

I'm really hoping they can get hypermiling experts like Wayne Gerdes (http://www.motherjones.com/politics/2007/01/guy-can-get-59-mpg-plain-old-accord-beat-punk and http://web.archive.org/web/20090226062953/http://post-gazette.com/pg/05220/550484.stm) who runs cleanmpg.com for a redo. I think w/the proper instrumentation, he might be able to achieve 2x the mileage.


brianric said:


> The 2012 Camry Hybrid has electronic air conditioning and water pump.


Yeah, all Toyota hybrids from the 04 model year and beyond have an electric AC compressor. AC works just fine when the engine's off.

On 2012+ model year Toyota hybrids, water pump become electric too so AC, power steering and water pump are all electric and there are no longer any "drive" belts.


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## midas (Jun 1, 2000)

There's a big difference between a car with auto stop/start and doing it yourself. Most cars require the car to be in park to engage the starter. So it's stop at the light, put car in park, turn off, wait for light to change, start car, put car in drive, move car. 

That's a lot to go through at every traffic light. A car that had the function is simply stop, wait, go.


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## cwerdna (Feb 22, 2001)

midas said:


> There's a big difference between a car with auto stop/start and doing it yourself. Most cars require the car to be in park to engage the starter. So it's stop at the light, put car in park, turn off, wait for light to change, start car, put car in drive, move car.
> 
> That's a lot to go through at every traffic light. A car that had the function is simply stop, wait, go.


Umm, non-hybrid automatics I've driven can be restarted while in neutral.

When I had my Z and learned about hypermiling, I'd FAS into a parking space (while still in D) and coast in. Once I'm stopped, I then shift to park.

Most automatic drivers pull into a space, stop, shift to park, then shut off the car. There's a whole bunch of wasted engine runtime in there.

Wayne talks about FASing into a parking spot at http://www.cleanmpg.com/forums/showthread.php?t=1510 under "Potential Parking". I generally tried that too, if I could find such a higher spot.

For the traffic light, if it looks almost certain they will have to stop, they should FAS then coast in D or N. Once they've stopped, from N, they can restart and go to D.

And yes, I have a Prius so I've been enjoying the engine seamlessly shutting down and starting up for over 7 years.  I've also driven other hybrids and some BMW 3-series cars w/auto start stop.


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## john4200 (Nov 1, 2009)

midas said:


> As always, this show is more about fun than true scientific proff.


Agreed. I have most of the same objections as you to their scientific method. Sometimes Jamie and Adam do a decent job, and sometimes they have some serious holes in their methods. Tory, Kari, and Grant almost always have serious holes in their methods -- they are mostly in it just for fun.


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## midas (Jun 1, 2000)

cwerdna said:


> Umm, non-hybrid automatics I've driven can be restarted while in neutral.


Most, but not all. Either way, it doesn't matter. I still find it very annoying and distracting. It's still a very different driving experience compared to an auto stop/start system.


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## RonDawg (Jan 12, 2006)

john4200 said:


> No, you missed the point. They were avoiding braking of any kind in order to keep their momentum up and save fuel. If you brake (with the brakes, or the engine) before going into a turn, then you need to use more fuel later to speed up again.


All that extra momentum won't do you any good if you can't stay planted on the road and end up crashing. A good hypermiler will know where he/she will have to slow down due to a tight corner, and let their speed naturally scrub off before they get to that point. It's really no different than anticipating stop lights and stop signs.


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## RonDawg (Jan 12, 2006)

Waldorf said:


> I'm currently driving a Hyundai Sonata Hybrid rental and find the engine cut-off at stops to be a bit unnerving. Starts right up like a golf cart, though.


I got used to it very quickly. I don't know about the Hyundai, but on the BMW there is an area on the tach to indicate if the engine is in "standby" as opposed to totally off (0 RPMs).



vertigo235 said:


> I assume you can turn it off. What about those times when you're in heavy traffic and Need the air conditioner while you're stopped?


On the BMW, yes you can turn it off. It's just above the starter button. If the car senses a need to run the engine for some reason (for example, heater usage) then it will do so.


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## RonDawg (Jan 12, 2006)

JYoung said:


> I'm not sure but I thought that I saw a CHP escort for Kari and Grant when they ran out of gas on the hypermile lap.
> Perhaps that prevented them from being victims of road rage.


Even with the CHP escort, a car still honked at Tory and (according to him) the female driver flipped him off.

45 MPH on any California freeway is just plain crazy unless everybody else is doing that speed. Many freeways that are posted at 55 still have traffic going by at 65+. Bay Bridge is posted 50 up to the temporary s-curve, and even then people are racing to that curve at 70, and going through the curve at higher than the posted 45 limit.


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## RonDawg (Jan 12, 2006)

Hank said:


> They prob could have shifted into neutral on the down slopes and idle the engine, but I know that's not generally legal.


It's not legal (certainly not in California where this was filmed) and quite dangerous if you let your speed get out of hand. It's even worse if you tried to control your speed by "riding" the brakes, because if they are overheated, they don't work at all.

From what I've been told on this very forum, on a modern fuel injected car if you downshift in order to control your speed on a downhill run, and don't put your foot on the gas, the engine is using zero fuel and is just one giant air pump powered by gravity, so you're not using any gas whatsoever.


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## RonDawg (Jan 12, 2006)

cwerdna said:


> For example, if they were approaching a red light, they should've shut off their engine at some point before the red light and just coasted to the intersection.


Problem with that is on a non-hybrid, you lose power braking and steering assist once the engine is off. Definitely not recommended on San Francisco's hilly streets.


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## john4200 (Nov 1, 2009)

RonDawg said:


> All that extra momentum won't do you any good if you can't stay planted on the road and end up crashing.


Really? If only I thought of that before my last crash.


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## Waldorf (Oct 4, 2002)

RonDawg said:


> I got used to it very quickly. I don't know about the Hyundai, but on the BMW there is an area on the tach to indicate if the engine is in "standby" as opposed to totally off (0 RPMs).


I have two BMW's as well as a "fun" car and you can really sense the engine running at stops. On this hybrid (and most others, I imagine), it's just kind of strange to be sitting there with no vibration coming through the car as if it died. And yes, like others have stated, the radio and a/c and everything will still run off battery, I'm just not sold on it yet as I feel disconnected from what's going on.

Has anyone tried the Honda CR-Z? I have read it has a 6-speed manual transmission option on the hybrid. Sounds interesting.


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## RonDawg (Jan 12, 2006)

john4200 said:


> Really? If only I thought of that before my last crash.


I'm not sure why you're being so snarky here.

Yes I can understand why a hypermiler doesn't want to slow down. But driving in real life conditions means eventually you have to slow, if not come to a stop. A hypermiler will anticipate these and try to let the car naturally scrub off speed before getting to such an area, rather than racing up to red lights and stop signs as I often see.

I was also only commenting on their assertion that their methods are legal and safe. 45 on a posted 65 is borderline rolling traffic hazard, and taking curves so fast that you cause the tires to squeal means you're on the verge of losing traction.

BTW I drive an all-electric car as my commuting car, so I'm acutely aware of these techniques myself. When your usable range per charge is only in the double digits, you learn to adapt real quick.


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## Waldorf (Oct 4, 2002)

This may be the source of the "double your mpg by hypermiling" myth as they mention getting 50mpg in a car rated for 25mpg

http://www.cnn.com/video/#/video/tech/2008/06/03/obrien.hypermiling.cnn

I saw another one similar to this where guys driving insights and priuses were getting over 100mpg but I haven't been able to source it yet.


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## john4200 (Nov 1, 2009)

RonDawg said:


> I'm not sure why you're being so snarky here.


You astound me with your lack of surety! 

Perhaps you will share some more words of wisdom. Do you suppose the gas mileage of a car will be lower if you drive it immediately after a crash? Do you think a crash you can walk away from is better than one where you end up in critical condition at the hospital? Is someone with a sense of humor more likely to survive a crash than someone lacking in that department?


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## RonDawg (Jan 12, 2006)

john4200 said:


> You astound me with your lack of surety!
> 
> Perhaps you will share some more words of wisdom. Do you suppose the gas mileage of a car will be lower if you drive it immediately after a crash? Do you think a crash you can walk away from is better than one where you end up in critical condition at the hospital? Is someone with a sense of humor more likely to survive a crash than someone lacking in that department?


Rather than continue to sidetrack this thread with any more futile attempts at rational discussion with you, I will just ignore you from now on.


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## RonDawg (Jan 12, 2006)

Waldorf said:


> Has anyone tried the Honda CR-Z? I have read it has a 6-speed manual transmission option on the hybrid. Sounds interesting.


A friend of mine test drove one and found it totally...underwhelming.


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## john4200 (Nov 1, 2009)

RonDawg said:


> Rather than continue to sidetrack this thread...


So, no more words of wisdom about how crashing is bad, then? 

(I would have gone with "no more running this thread out of control" myself)


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## cwerdna (Feb 22, 2001)

RonDawg said:


> Problem with that is on a non-hybrid, you lose power braking and steering assist once the engine is off. Definitely not recommended on San Francisco's hilly streets.


Well, you get at least 1 or 2 presses of the brake power assist w/the engine off, but yeah, there is the loss of power steering.

To me, it'd be not too terrible if to shut off the engine when approaching a traffic light that's only straight ahead and you know how well the car's speed scrubs off when the engine's off, so that you don't need the brakes at all/much.



Waldorf said:


> I have two BMW's as well as a "fun" car and you can really sense the engine running at stops. On this hybrid (and most others, I imagine), it's just kind of strange to be sitting there with no vibration coming through the car as if it died.


It's not too weird anymore, but then again, I've had my Prius for over 7 years now. For me, it drives me nuts to be in a regular car w/the stupid engine idling away when stopped. And, it irks me greatly to be in such a car and using the brakes, to know _all_ that kinetic energy is being thrown away as heat and brake dust.

The BMW 3-series cars that I drove w/auto-start stop have a comical restart noise. I heard it from other BMWs on the test drive loop. It was just hilarious to me (from someone having driven hybrids from Toyota, Nissan (well it's Toyota HSD based), Ford, Honda and Kia).


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## cwerdna (Feb 22, 2001)

Waldorf said:


> Has anyone tried the Honda CR-Z? I have read it has a 6-speed manual transmission option on the hybrid. Sounds interesting.


No, but its fuel economy are disappointing along w/its straight line performance.

See how it did in CR's FE tests at http://www.consumerreports.org/cro/2012/02/the-most-fuel-efficient-cars/index.htm?loginMethod=auto. It scored too low in their tests to recommend.

In their tests, it did 0-60 in 9.3 seconds so ok it's faster than a Prius but a Camry Hybrid blows it away (http://www.consumerreports.org/cro/2012/01/fuel-economy-vs-performance/index.htm). My mom's Altima Hybrid got identical acceleration times to the '12+ Camry Hybrid (not surprising since they're only 2 hp apart).

I also wouldn't trust the battery longevity of any Honda hybrid. Their track record is not particularly good. It's unfortunate that Honda's not putting up a better fight against Toyota.


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## Waldorf (Oct 4, 2002)

Waldorf said:


> I saw another one similar to this where guys driving insights and priuses were getting over 100mpg but I haven't been able to source it yet.


This is in the same vein as what I was thinking of.. he averaged over 106 mpg on a tank:

http://www.startribune.com/lifestyle/22053954.html

I think he may have been part of a group of people I saw on another episode of 20/20 or Dateline or something else, but I haven't been able to find it.


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## voripteth (Apr 9, 2003)

We regularly get 100+ MPG but then again we have a Volt.


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## verdugan (Sep 9, 2003)

RonDawg said:


> Rather than continue to sidetrack this thread with any more futile attempts at rational discussion with you, I will just ignore you from now on.


The ignore list is a powerful thing. Use it. Shout if you have quesitons on how it works.


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## Waldorf (Oct 4, 2002)

voripteth said:


> We regularly get 100+ MPG but then again we have a Volt.


How long have you had it / how is it so far? The Volt is a certainly different beast... I remember reading that Jay Leno had 11,000 miles on his Volt and still had half the original tank of gas.


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## Flop (Dec 2, 2005)

secondclaw said:


> I was actually hoping they would do another HM thing - accelerating 5mph above posted speed limit then coasting down ... I've always been curious if that method actually does anything but been too lazy to try it myself. I suppose they don't want to break the law on TV, even if its a 5mph over limit.


I can't imagine how that would save any gas. Since the drag on the car is more or less constant at those speeds, it seems you would burn more gas to produce the amount of energy required to accelerate the mass of the car and to overcome drag from air resistance and tire friction than you could possibly save by coasting back down. It's been many years since I took physics, but I believe the power required to overcome aerodynamic drag increases as the _cube_ of the velocity (it might be the square not cube... it's been a while). At high speeds (say going from 65 to 70) that power difference won't be a lot, but it's still additional power required, and you still have to add the power required to accelerate the mass of the car as well.


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## Hank (May 31, 2000)

It's the square.


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## john4200 (Nov 1, 2009)

Flop said:


> It's been many years since I took physics, but I believe the power required to overcome aerodynamic drag increases as the _cube_ of the velocity (it might be the square not cube... it's been a while). At high speeds (say going from 65 to 70) that power difference won't be a lot, but it's still additional power required, and you still have to add the power required to accelerate the mass of the car as well.


The third power is correct.

You are certainly correct about speeding up and coasting being an inefficient technique in general.

Although I can imagine a more sophisticated technique, that slows down a bit when there is a headwind and speeds up a bit when there is a tailwind, could be effective. It would probably need electronic automation to do it well -- a digital speedometer or GPS to monitor ground speed, an anemometer to monitor airspeed, and a computer to compare them and determine if there is sufficient headwind or tailwind to merit a throttle change.


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## john4200 (Nov 1, 2009)

Hank said:


> It's the square.


No. The drag is proportional to the second power of velocity, but the power required to offset the drag is proportional to the third power of velocity:

P = F * v = ( k * v^2 ) * v = k * v^3


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## RonDawg (Jan 12, 2006)

verdugan said:


> The ignore list is a powerful thing. Use it. Shout if you have quesitons on how it works.


I'm well aware of how it works, thank you.


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## kdmorse (Jan 29, 2001)

john4200 said:


> You are certainly correct about speeding up and coasting being an inefficient technique in general.


I wonder about the hypothetical engine/transmission combination that is more wasteful maintaining speed than accelerating. Where accelerating briskly and efficiently for 10 seconds then idling for 50, is more efficient than maintaining speed at an inefficient RPM for 60 seconds.

Not that it would ever be the case in a modern automobile, but I wonder if it was at least partially the case for some model of 60's/70's vehicles, enough so to generate the idea....


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## alansh (Jan 3, 2003)

The speed up and slow down thing is "pulse and glide", and may be specific to the Prius or at least hybrids in general.

The idea of the hybrid is to keep the internal combustion engine at the most efficient point in its power curve, regardless of the amount of power you actually need. So if the ICE is generating too much power, the excess goes into the battery and is used when you need more power than the engine is creating.

There are losses in the process, so pulse and glide aims to use 100% of the engine's output for acceleration, rather than having an excess that goes into the battery. You accelerate to above your target speed, with the goal of none going into the battery. Once you're above your target speed you coast, with the engine turning off automatically. Once you've slowed enough, you repeat.

You could theoretically do this in a non-hybrid, but the shifting into neutral and turning off the engine for the glide part is going to be a problem.


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## john4200 (Nov 1, 2009)

alansh said:


> There are losses in the process, so pulse and glide aims to use 100% of the engine's output for acceleration, rather than having an excess that goes into the battery. You accelerate to above your target speed, with the goal of none going into the battery. Once you're above your target speed you coast, with the engine turning off automatically. Once you've slowed enough, you repeat.


While that technique may be able to increase mileage in city driving, I thought we were discussing freeway driving, where such a technique would surely hurt your mileage, for reasons already explained by Flop.


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## alansh (Jan 3, 2003)

Yes, pulse and glide is typically used around 40mph or less on the Prius. At those speeds, the difference in the power curve is enough to make a bigger difference than the wind resistance. Above 40mph, the gen II Prius has to run the gas engine because of the way the gearing between the electric and gas motors is done, so the engine never really shuts off. I haven't really looked into the difference for the gen III.

Anyway, as I mentioned, this would be really difficult to do on a non-hybrid.


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## cwerdna (Feb 22, 2001)

alansh said:


> Yes, pulse and glide is typically used around 40mph or less on the Prius. At those speeds, the difference in the power curve is enough to make a bigger difference than the wind resistance. Above 40mph, the gen II Prius has to run the gas engine because of the way the gearing between the electric and gas motors is done, so the engine never really shuts off.* I haven't really looked into the difference for the gen III.*


On Gen 3 Prius, I believe the max speed at which you can drive w/the ICE not turning is ~46 mph, rather than 40 or 41 mph.

There's also the efficiency map at http://priuschat.com/threads/2010-prius-2zr-fxe-engine-efficiency-map.62586/. 1NZ-FXE is the 1.5L engine used in the Gen 2 (04-09) Prius while the 2ZR-FXE is the 1.8L engine used in the 2010+ Prius liftbacks.


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## brianric (Aug 29, 2002)

alansh said:


> You accelerate to above your target speed, with the goal of none going into the battery. Once you're above your target speed you coast, with the engine turning off automatically. Once you've slowed enough, you repeat.


While you are coasting you are charging the battery.


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## cwerdna (Feb 22, 2001)

brianric said:


> While you are coasting you are charging the battery.


Yeah, you're not supposed to coast (at least if you're in D).

Normally one should glide. See http://www.cleanmpg.com/forums/showthread.php?t=1224. You're supposed to dead band so that no arrows are going in/out anywhere (see Proper Glide picture).

Basically, you want to minimize the power flow in or out of the battery as doing so incurs losses.


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## Flop (Dec 2, 2005)

This all sounds like way too much work and hassle for me 

I just get in my car and drive. When I want better mileage I fill up with 100% gas and skip that ethanol crap


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## pmyers (Jan 4, 2001)

Flop said:


> This all sounds like way too much work and hassle for me


I agree....and who wants to "work" that hard while driving?


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## voripteth (Apr 9, 2003)

Waldorf said:


> How long have you had it / how is it so far? The Volt is a certainly different beast... I remember reading that Jay Leno had 11,000 miles on his Volt and still had half the original tank of gas.


We've had it over a year. If you have a daily commute shorter than 40 miles you almost never use gas. For those longer weekend trips the gas generator kicks in to supply another 300+ miles. While the gas motor is running we get around 40 MPG. The 100 MPG figure is combining it with our battery use.

Many of the hypermiler tricks are also used to wring as much range as possible from the battery. I'm surprised they didn't over-inflate the tires to 40 lbs which is a common practice.


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## RonDawg (Jan 12, 2006)

Flop said:


> This all sounds like way too much work and hassle for me
> 
> I just get in my car and drive. When I want better mileage I fill up with 100% gas and skip that ethanol crap





pmyers said:


> I agree....and who wants to "work" that hard while driving?


Once you get used to driving more economically, much of it just becomes second nature. For example, once I got an electric car, I couldn't understand why people have to "race" up to an already red light or stop sign. You know you have to stop...why use extra gas just to get you there a second early? You also use up brake pads faster because now you have to scrub off all that extra speed.

Keeping speeds down really isn't hard work either. I've driven at 65 MPH on a posted 65 MPH freeway...even in LA, it's quite possible to do this 

Some of the techniques like shutting off the engine at traffic lights (without the use of auto stop/start) I do agree is more work, and when I drive my ICEV I only do this when I know I'll be stopped for quite some time, such as at a train crossing.


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## verdugan (Sep 9, 2003)

RonDawg said:


> For example, once I got an electric car, I couldn't understand why people have to "race" up to an already red light or stop sign. You know you have to stop...why use extra gas just to get you there a second early? You also use up brake pads faster because now you have to scrub off all that extra speed.


So you're saying that you used to do that until you got an Electric vehicle?

I have a regular gas car and I can't for the life of me understand why people do that. It always reminds me of the scene from "Meet the Parents" when Ben Stiller and Robert DeNiro are racing from light to light.


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## JYoung (Jan 16, 2002)

RonDawg said:


> Keeping speeds down really isn't hard work either. I've driven at 65 MPH on a posted 65 MPH freeway...even in LA, it's quite possible to do this


Yes, but have you driven at 45 MPH on a posted 65 MPH freeway not due to traffic?


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## squint (Jun 15, 2008)

cwerdna said:


> On Gen 3 Prius, I believe the max speed at which you can drive w/the ICE not turning is ~46 mph, rather than 40 or 41 mph.
> 
> There's also the efficiency map at http://priuschat.com/threads/2010-prius-2zr-fxe-engine-efficiency-map.62586/. 1NZ-FXE is the 1.5L engine used in the Gen 2 (04-09) Prius while the 2ZR-FXE is the 1.8L engine used in the 2010+ Prius liftbacks.


I should emphasize the distinction here between the ICE turning (components rotating) and the ICE turning on. I believe the Gen 3 Prius and Camry equivalent can have the ICE off > 46 mph but no gas is being used.


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## RonDawg (Jan 12, 2006)

verdugan said:


> So you're saying that you used to do that until you got an Electric vehicle?
> 
> I have a regular gas car and I can't for the life of me understand why people do that. It always reminds me of the scene from "Meet the Parents" when Ben Stiller and Robert DeNiro are racing from light to light.


Actually, yes, but not as extreme as some.

An example is I will drive along a posted 40 MPH boulevard at the posted limit. I see the light about 2 blocks ahead is red, or just turned red, so I'll let off the accelerator. A block away my speed has dropped to about 30 MPH as I'm trying to use as little brake as possible, but people behind me will get impatient and rocket past me and still get stuck at the red light.

Before I got the electric car I wouldn't let off the gas until I was less than a block from the light. But some folks will not let off until fairly close to the light, and then they have to use quite a bit of braking.

Then those same folks are left wondering when as soon as the light is green, I can accelerate fairly quickly away from a stop with relatively little accelerator pedal pressure, thanks to having 210 lb-ft of torque available throughout the motor's rev range.


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## RonDawg (Jan 12, 2006)

JYoung said:


> Yes, but have you driven at 45 MPH on a posted 65 MPH freeway not due to traffic?


Yes, when it's extremely foggy or rainy. But when the weather's that bad, I prefer to stay off the roads completely if I can.


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## midas (Jun 1, 2000)

RonDawg said:


> Actually, yes, but not as extreme as some.
> 
> An example is I will drive along a posted 40 MPH boulevard at the posted limit. I see the light about 2 blocks ahead is red, or just turned red, so I'll let off the accelerator. A block away my speed has dropped to about 30 MPH as I'm trying to use as little brake as possible, but people behind me will get impatient and rocket past me and still get stuck at the red light.


You've also go to keep in mind that many people just aren't very good drivers. They barely look past the end of their hood. I've seen people get stuck in lane closures where there was a big flashing sign that could be seen for half a mile in relatively light traffic. But they don't see it until they are almost on top of the first traffic horse.

Those people wouldn't notice that the traffic light ahead was red until they got within a few hundred feet.


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## Idearat (Nov 26, 2000)

midas said:


> You've also go to keep in mind that many people just aren't very good drivers. They barely look past the end of their hood. I've seen people get stuck in lane closures where there was a big flashing sign that could be seen for half a mile in relatively light traffic. But they don't see it until they are almost on top of the first traffic horse.
> 
> Those people wouldn't notice that the traffic light ahead was red until they got within a few hundred feet.


I see it more as people who slow down a 2 blocks away assume they're the only one on the road. They ignore traffic triggered lights (if you're not close it won't go green). If you're in the right lane you can be blocking the guy who is going to turn right on red. If you're in the left lane (if there is one ) you're blocking the guy who wants to get into the left turn lane. If the left turn signal is traffic operated you just made that person wait through an entire cycle, with his engine idling. (There's one intersection in my town where people do that to me all the time)

The descriptions about clueless drivers above only mention inattentiveness to the road, but doesn't mention other people who may not be going where you expect. Forcing everyone to drive the way you are is just rude. Driving slower than normal so that traffic is bunching up behind you is as bad as tailgating someone to make them speed up.

Basically driving like that with _anyone_ behind you is the same as sticking your middle finger out at them.


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## verdugan (Sep 9, 2003)

RonDawg said:


> Actually, yes, but not as extreme as some.
> 
> An example is I will drive along a posted 40 MPH boulevard at the posted limit. I see the light about 2 blocks ahead is red, or just turned red, so I'll let off the accelerator. A block away my speed has dropped to about 30 MPH as I'm trying to use as little brake as possible, but people behind me will get impatient and rocket past me and still get stuck at the red light.
> 
> ...


I drive my gas car like you drive your electric car.

I always get a smile on my face those times when (by luck) I timed my coasting well so that I get off the intersection faster than the people who accelerated to the intersection and had to do a full stop.


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## pmyers (Jan 4, 2001)

I'd lump hypermilers in with the crazy couponers


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## RonDawg (Jan 12, 2006)

verdugan said:


> I drive my gas car like you drive your electric car.


You may not want to visit Gilroy then!


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## ferrumpneuma (Jun 1, 2006)

RonDawg said:


> Actually, yes, but not as extreme as some.
> 
> An example is I will drive along a posted 40 MPH boulevard at the posted limit. I see the light about 2 blocks ahead is red, or just turned red, so I'll let off the accelerator. A block away my speed has dropped to about 30 MPH as I'm trying to use as little brake as possible, but people behind me will get impatient and rocket past me and still get stuck at the red light.
> 
> ...





verdugan said:


> I drive my gas car like you drive your electric car.
> 
> I always get a smile on my face those times when (by luck) I timed my coasting well so that I get off the intersection faster than the people who accelerated to the intersection and had to do a full stop.


My grand dad used to do this kind of thing. I think it was a holdover from when he was a truck driver. When he got old he took it to the extreme. He would slow down to single digit speed if the traffic signal on the horizon was red and floor it when it turned green.


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## verdugan (Sep 9, 2003)

ferrumpneuma said:


> My grand dad used to do this kind of thing. I think it was a holdover from when he was a truck driver. When he got old he took it to the extreme. He would slow down to single digit speed if the traffic signal on the horizon was red and floor it when it turned green.


I don't take it to that extreme. People don't honk when I'm doing it.

I caught a little bit of the Dragnet movie (Dan Aykroyd and Tom Hanks.) Tom Hank's character was making fun of Dan Aykroid's character for driving so slow. He said he was driving 8 mph below the speed limit to save the taxpayer's some gas money.


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## RonDawg (Jan 12, 2006)

verdugan said:


> I caught a little bit of the Dragnet movie (Dan Aykroyd and Tom Hanks.) Tom Hank's character was making fun of Dan Aykroid's character for driving so slow. He said he was driving 8 mph below the speed limit to save the taxpayer's some gas money.


IIRC he also drove a Yugo, "the pinnacle of Serbo-Croatian technology" in the words of the Joe Friday character.


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## justen_m (Jan 15, 2004)

secondclaw said:


> I was actually hoping they would do another HM thing - accelerating 5mph above posted speed limit then coasting down ...





Flop said:


> I can't imagine how that would save any gas. Since the drag on the car is more or less constant at those speeds, it seems you would burn more gas to produce the amount of energy required to accelerate the mass of the car and to overcome drag from air resistance and tire friction than you could possibly save by coasting back down. It's been many years since I took physics, but I believe the power required to overcome aerodynamic drag increases as the _cube_ of the velocity (it might be the square not cube... it's been a while). At high speeds (say going from 65 to 70) that power difference won't be a lot, but it's still additional power required, and you still have to add the power required to accelerate the mass of the car as well.


I think hypermilers are erroneously taking results from world record holders getting 10,000+ MPG in super specialized vehicles averaging 15mph, for example (from the Shell Eco-marathon) from wiki http://en.wikipedia.org/wiki/Eco-marathon :
"The top performing vehicles are specially designed for high efficiency. Some vehicles use a coast/burn technique whereby they briefly accelerate from 10 to 20 mph (from 16 to 32 km/h) and then switch the engine off and coast for approximately 2 minutes until the speed drops back down to 10 mph (16 km/h). This process is repeated resulting in average speed of 15 mph for the course. Typically the vehicles have:
Automobile drag coefficients (Cd) < 0.1
Rolling resistance coefficients < 0.0015
Weight without driver of < 45 kg
Engine efficiency of < 200 s.f.c. (cc/bhp/hr)
The vehicles are highly specialized and optimized for the event and are not intended for everyday use. The designs represent what can be achieved with current technology and offer a glimpse into the future of car design based on minimal environmental impact in a world with reduced oil reserves. The work of the participants can be used to show ways manufacturers could redesign their products."

For real life products, these results aren't really applicable. A little bit of knowledge is dangerous. Or in this case, just stupid.

Regarding other parts of this episode... the SynDavers were cool. Were they SynDavers (from that company) which run $40,000 each? Or from some other company? $80k to destroy a couple of dummies for the episode would seem to have reached a new budget threshold for the Mythbusters.


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## LoadStar (Jul 24, 2001)

justen_m said:


> Regarding other parts of this episode... the SynDavers were cool. Were they SynDavers (from that company) which run $40,000 each? Or from some other company? $80k to destroy a couple of dummies for the episode would seem to have reached a new budget threshold for the Mythbusters.


Right company, I think, but wrong model. I'm pretty sure they're using the "Muscled Full Body" Anatomy Model from that company, which retails for $13k... and I'd bet they're getting a (much) better deal than retail pricing.


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## cwerdna (Feb 22, 2001)

justen_m said:


> I think hypermilers are erroneously taking results from world record holders getting 10,000+ MPG in super specialized vehicles averaging 15mph, for example (from the Shell Eco-marathon) from wiki


I'm not so sure about that. They've achieved records at Hybridfest (I don't have the links handy) and records like http://www.cleanmpg.com/forums/showthread.php?t=2783 (2,254.4 miles on 13.72 gallons = 164.315 mpg on a Gen 1 Insight).

Also see my links in post 27.

A Priuschatter did this graph: http://priuschat.com/threads/updated-mpg-vs-mph-chart.64296/. I found http://www.cnn.com/video/standard.html#/video/international/2008/01/13/lustout.hack.hybrid.cnn (from http://priuschat.com/threads/lifetime-mileage-report-by-japanese-hypermiler.73638/#post-1363174) on Japanese hypermilers. That video on the shogun is a bit misleading, his Prius wasn't a plug-in (to charge the battery). It was for the engine block heater: to cut or eliminate warm up time, which is a period of high fuel consumption.

I was going to post a video I found on Wayne Gerdes on CNN, but apparently Waldorf in post 40 beat me to it. I hadn't seen it before until today.

BTW, personally, I think the hypermiling behavior of folks like Wayne and the shogun are extreme and not something I'd recommend. There are a few things one can do that bother nobody and are simple (e.g. face out parking, FASing into a parking spot, combining trips, going to the furthest destination first, keeping your speed down a bit, etc.)

But, it does irk me to see some of the bad driving people have talked about (e.g. accelerating/racing to red lights). I've also seen people idling their cars for an extended period of time for no apparent reason in parking lots (not cold, not hot out).

I've also seen enormous lines (dozen+ cars long) at certain fast food places like In-N-Out and almost none of them are hybrids, PHEVs, EVs nor any car w/auto stop/start systems. Some of them are BRoD class SUVs. Hope none of those folks are complaining of high gas prices or poor gas miles. 

RonDawg summed up people's unwillingness to change their own driving behaviors even during "high" gas prices pretty well at http://www.tivocommunity.com/tivo-vb/showthread.php?p=9625954#post9625954.


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## RonDawg (Jan 12, 2006)

cwerdna said:


> But, it does irk me to see some of the bad driving people have talked about (e.g. accelerating/racing to red lights)...RonDawg summed up people's unwillingness to change their own driving behaviors even during "high" gas prices pretty well at http://www.tivocommunity.com/tivo-vb/showthread.php?p=9625954#post9625954.


I experienced a couple of examples of such behavior tonight. First, as I'm driving down a local 65 MPH posted freeway at, shocker, 65 MPH  I noticed a Honda Civic tailgating me. Never mind there were three open lanes to my left, she insisted on following too closely.

Rather than do a "brake check" that is so tempting, but so dangerous, I do the recommended thing and simply let my speed scrub off naturally. I put the car in "Eco" mode and let off the accelerator, and let the car's regen naturally slow the car to about 55 MPH. She finally got the point and went around me, at which time I found out her particular Civic was a hybrid model. Why buy the slower model that supposedly gets better mileage if you're going to rocket around me at 80 MPH 

And no, she didn't need to exit at the next ramp, so nobody was "holding her up." It's her failure to drive with her head out of her anus and either maintain a safe following distance, or use the other three lanes to pass me. G-d forbid I actually obey the speed limit.

Second one is the same posted 40 MPH boulevard on which my stated driving style seems to have gotten a rather strong reaction earlier. Traffic is going about 45 MPH, and yes I am slowing like I mentioned earlier. But frankly, so is everybody else due to the amount of traffic. That still didn't stop some kid (no older than 21 from what I can tell, so likely parents bought the car for him) in a new Toyobaru driving like a jackazz and passing everybody. Or trying to anyway, because the traffic signal gods were not in his favor. And guess who often ended up in the lane next to him, or right behind him, at EVERY single traffic light for several miles


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## pteronaut (Dec 26, 2009)

Could it be that the first a-hat was (in her mind) trying to take advantage of the spoiled air behind you to reduce her vehicle's fuel consumption? (Not that it excuses the tailgating)


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## eddyj (Jun 20, 2002)

pteronaut said:


> Could it be that the first a-hat was (in her mind) trying to take advantage of the spoiled air behind you to reduce her vehicle's fuel consumption? (Not that it excuses the tailgating)


That was one of the techniques they did not use, along with rolling stops.


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## Flop (Dec 2, 2005)

eddyj said:


> That was one of the techniques they did not use, along with rolling stops.


They tested drafting on a previous episode I believe and determined you have to be extremely close (like 2 or 3 feet close) behind the other vehicle for it to be effective. It's been a while since I saw that episode, but I remember them having to put markers on the windshield of the test car and on the back of the lead vehicle for the test driver to line up in order to maintain distance since it was so close.


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## RonDawg (Jan 12, 2006)

Yeah, in that epsiode Grant was a couple of feet from the back bumper of that semi. He later said it was absolutely nerve-wracking.


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## pteronaut (Dec 26, 2009)

I believe that they did see some improvement even before they got that close in that episode. Although I'd doubt that drafting a car would be effective until you got as close as NASCAR racers do.


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## MarkofT (Jul 27, 2001)

RonDawg said:


> She finally got the point and went around me, at which time I found out her particular Civic was a hybrid model. Why buy the slower model that supposedly gets better mileage if you're going to rocket around me at 80 MPH


I generally cruised around at or a bit above the 80 MPH speed limit in my 1st gen Insight. From my experience with that one specific hybrid, they are sportier then people think because they get obsessed with maximizing their MPG as opposed to driving. Even with the way I drove it, I still averaged 40 MPG.

In the drafting episode, they saw improvements at all distances with staggering improvements at distances closer then most people feel comfortable parking.

And I can vouch that you get a decent MPG bump with just about any drafting. But you are not drafting until you can feel your vehicle entering the turbulent wake behind another vehicle. In my F250, if I can get behind a semi, I'll see a 15% - 20% increase in MPG. In the BMW 323i we had, it would get a much bigger boost behind a semi at 80 MPH due to the relative size of the holes needed by each vehicle.

To get into the draft behind a Leaf at 60 MPH, you would have to get quite close indeed due to the small hole in the air it makes plus the extra aerodynamics they put into smoothing the overall airflow that results in a much smaller draft. I doubt anyone other then a bicyclist would see any benefit at all from trying.


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## Dirk Legume (Nov 29, 2004)

I have a Sonata hybrid which I believe is rated at 38 by the EPA. Just by doing the things that RonDawg mentioned, I get much better than that. My last tank was 43 MPG and that included a trip, during which I had to drive 70 MPH for about 60 miles. It isn't hard to do and after a while, it just becomes second nature.


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## chocophile (Dec 27, 2007)

RonDawg said:


> It's not legal (certainly not in California where this was filmed) and quite dangerous if you let your speed get out of hand. It's even worse if you tried to control your speed by "riding" the brakes, because if they are overheated, they don't work at all.
> 
> From what I've been told on this very forum, on a modern fuel injected car if you downshift in order to control your speed on a downhill run, and don't put your foot on the gas, the engine is using zero fuel and is just one giant air pump powered by gravity, so you're not using any gas whatsoever.


RD is correct. Datalogging has been used to show that modern computer controlled, fuel injected engines use no fuel whatsoever when coasting in gear. Coasting in neutral with the engine running DOES use (some) fuel.

Keep the engine running folks, so that your power brakes and power steering work if/when needed!


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