# Speed up...coast down to slower speed **URL:** \ **Category:** The Show\ **Created:** [July 16, 2011, 3:11pm UTC](https://community.cartalk.com/t/speed-up-coast-down-to-slower-speed/66296 "2011-07-16T15:11:25Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![retrophyt](https://avatars.discourse-cdn.com/v4/letter/r/bc79bd/32.png) [@retrophyt](https://community.cartalk.com/u/retrophyt)\ **Post date:** [July 16, 2011, 3:11pm UTC](https://community.cartalk.com/t/speed-up-coast-down-to-slower-speed/66296/1 "2011-07-16T15:11:25Z")
Who cares about the possible gas mileage? Have you ever tried to follow someone doing this? I feel on the verge of road rage every time it happens. You certainly can’t use cruise control. It is even difficult to pass on 4 lane, almost impossible on a 2 lane. ---
**Author:** ![mcparadise](https://sea1.discourse-cdn.com/cartalk/user_avatar/community.cartalk.com/mcparadise/32/32_2.png) [@mcparadise](https://community.cartalk.com/u/mcparadise)\ **Post date:** [July 16, 2011, 3:14pm UTC](https://community.cartalk.com/t/speed-up-coast-down-to-slower-speed/66296/2 "2011-07-16T15:14:31Z")
??? ---
**Author:** ![bscar](https://sea1.discourse-cdn.com/cartalk/user_avatar/community.cartalk.com/bscar/32/34_2.png) [@bscar](https://community.cartalk.com/u/bscar)\ **Post date:** [July 16, 2011, 4:01pm UTC](https://community.cartalk.com/t/speed-up-coast-down-to-slower-speed/66296/3 "2011-07-16T16:01:46Z")
I believe he’s talking about the people on the highway who can’t figure out if they want to do 55 or 65 or even 45 in the 55mph zone(as an example). ---
**Author:** ![bing](https://sea1.discourse-cdn.com/cartalk/user_avatar/community.cartalk.com/bing/32/4619_2.png) [@bing](https://community.cartalk.com/u/bing)\ **Post date:** [July 16, 2011, 5:50pm UTC](https://community.cartalk.com/t/speed-up-coast-down-to-slower-speed/66296/4 "2011-07-16T17:50:49Z")
First off I think you need some anger management techniques. On the other hand, people are so silly and worry about saving two dollars on fuel but will spend $40 on the golf course and think nothing of it. ---
**Author:** ![CGB\_Philly](https://avatars.discourse-cdn.com/v4/letter/c/df705f/32.png) [@CGB\_Philly](https://community.cartalk.com/u/CGB_Philly)\ **Post date:** [July 17, 2011, 12:11am UTC](https://community.cartalk.com/t/speed-up-coast-down-to-slower-speed/66296/5 "2011-07-17T00:11:19Z")
Not the right kind of scientist for an expert opinion. But my (many) engineer relatives brought me up to keep a mileage log, so I can offer experience. (BTW: Thanks for releasing me from the mileage log. Maybe it was useful in 1930.) Driving my Accord around 40 mph in the Colorado mountains – which go up and down naturally – I always got much better mileage than “on the flat”. Also, Ford seems to think this is true: my new MKX claims that it uses this tactic to improve its (unimpressive) gas mileage. ---
**Author:** ![Lin](https://avatars.discourse-cdn.com/v4/letter/l/13edae/32.png) [@Lin](https://community.cartalk.com/u/Lin)\ **Post date:** [July 17, 2011, 4:09pm UTC](https://community.cartalk.com/t/speed-up-coast-down-to-slower-speed/66296/6 "2011-07-17T16:09:11Z")
``` You don’t need equations to show that the husband is mistaken. Once both husband and wife have reached target velocity, say the speed limit, wife needs only enough force (think gas) to overcome friction, wind resistance, etc. Husband needs this plus extra force for two episodes of acceleration: once to get up to a higher speed, plus another acceleration to get back up to the speed limit after coasting. ``` ---
**Author:** ![mariaita](https://avatars.discourse-cdn.com/v4/letter/m/34f0e0/32.png) [@mariaita](https://community.cartalk.com/u/mariaita)\ **Post date:** [July 17, 2011, 7:15pm UTC](https://community.cartalk.com/t/speed-up-coast-down-to-slower-speed/66296/7 "2011-07-17T19:15:35Z")
The reason this dude should stop doing this stupid stuff is out of mercy for those of us trying to maintain an even speed on the highway. It’s dangerous and really obnoxious. This kind of behavior is on my top list of annoying things people do - up there with not using turn signals, driving like it’s a video game, and speeding up as you try to pass. ---
**Author:** ![bobandbrigit](https://avatars.discourse-cdn.com/v4/letter/b/ecd19e/32.png) [@bobandbrigit](https://community.cartalk.com/u/bobandbrigit)\ **Post date:** [July 17, 2011, 7:41pm UTC](https://community.cartalk.com/t/speed-up-coast-down-to-slower-speed/66296/8 "2011-07-17T19:41:15Z")
The formula for this is the Second Law of Thermodynamics (which basically says you can’t break even, the First Law says you can’t win and the Third says you can’t get out of the game.) That being said, back in the old days ('50s & 60s) when VW economy contests were held there was a technique similar to this - get up to speed, hold the accelerator pedal steady, depress the clutch and turn the ignition off. Let the car coast down almost to lugging speed, turn on the ignition and engage the clutch holding the pedal very steady so as not to cause the accelerator pump in the carb to shoot in a charge of gas which was far more wasteful than Second Law losses. ---
**Author:** ![waterboy](https://sea1.discourse-cdn.com/cartalk/user_avatar/community.cartalk.com/waterboy/32/764_2.png) [@waterboy](https://community.cartalk.com/u/waterboy)\ **Post date:** [July 17, 2011, 8:50pm UTC](https://community.cartalk.com/t/speed-up-coast-down-to-slower-speed/66296/9 "2011-07-17T20:50:49Z")
My wife does that kind of driving, living in New your City I think demands it. To me it is a logical extension of jackrabbit starts reducing gas mileage. ---
**Author:** ![B.L.E](https://avatars.discourse-cdn.com/v4/letter/b/848f3c/32.png) [@B.L.E](https://community.cartalk.com/u/B.L.E)\ **Post date:** [July 19, 2011, 11:43am UTC](https://community.cartalk.com/t/speed-up-coast-down-to-slower-speed/66296/10 "2011-07-19T11:43:48Z")
“Pulse and glide” works because engines become very inefficient at light loads. The idea is to make the engine produce a high power intermittently instead of a low power continuously. If the speed delta is too large, you can actually get worse gas mileage using this technique. You end up covering the majority of the distance at a speed higher than your average speed yet you spend the majority of your time at a speed lower than your average speed. At 70 mph, your engine is already loaded enough to make it reasonably efficient so there is little point in doing it on the freeway. Where pulse and glide really pays off is on roads where you can only go around 40mph or less. These roads tend to have so many curves and hills that you can pulse and glide without other drivers even being aware you are doing it. You pulse on the straight parts of a road and then you glide towards a turn that you need to slow down for anyway, or you pulse up a hill and then glide downhill which results in a nearly steady speed. ---
**Author:** ![Floyd](https://avatars.discourse-cdn.com/v4/letter/f/439d5e/32.png) [@Floyd](https://community.cartalk.com/u/Floyd)\ **Post date:** [July 20, 2011, 9:01pm UTC](https://community.cartalk.com/t/speed-up-coast-down-to-slower-speed/66296/11 "2011-07-20T21:01:41Z")
I think Einstein’s formula E=mc2 explains it. Squaring (c, the higher speed) more than covers any saving by squaring (c the lower speed). EX: Lets compare up to 6mph and down to 4mph with an average 5 mph. 6X6=36 + 4X4=16. Add 36 + 16 =52, divide by 2 to get an average of 26 for the multiplier in the formula. If he ran a constant 5 mph, then 5 squared is 25 for the multiplier. Hence a saving driving a constant speed. (This would give the same results for driving up and down for 60 & 40 mph compared to a steady 50mph. ---
**Author:** ![the\_same\_mountainbike](https://avatars.discourse-cdn.com/v4/letter/t/c89c15/32.png) [@the\_same\_mountainbike](https://community.cartalk.com/u/the_same_mountainbike)\ **Post date:** [July 20, 2011, 10:03pm UTC](https://community.cartalk.com/t/speed-up-coast-down-to-slower-speed/66296/12 "2011-07-20T22:03:10Z")
Occasionally I’ll get behind someone who constantly slows down and then speeds up and it drives me nuts. And the other cars I was lined up with behind this person didn’t act too pleased either. This is just plain poor driving habits. No excuses accepted. ---
**Author:** ![bscar](https://sea1.discourse-cdn.com/cartalk/user_avatar/community.cartalk.com/bscar/32/34_2.png) [@bscar](https://community.cartalk.com/u/bscar)\ **Post date:** [July 21, 2011, 12:30am UTC](https://community.cartalk.com/t/speed-up-coast-down-to-slower-speed/66296/13 "2011-07-21T00:30:36Z")
I agree. They might think they’re doing everyone a favor by doing that, but you never know when some nutjob will swerve around them, cut them off, then cause an accident. ---
**Author:** ![piter\_devries](https://avatars.discourse-cdn.com/v4/letter/p/aca169/32.png) [@piter\_devries](https://community.cartalk.com/u/piter_devries)\ **Post date:** [July 21, 2011, 1:31pm UTC](https://community.cartalk.com/t/speed-up-coast-down-to-slower-speed/66296/14 "2011-07-21T13:31:02Z")
“_Floyd July 20 Report I think Einstein’s formula E=mc2 explains it._” Are you _trying_ to get people not to listen to you? ---
**Author:** ![MikeInNH](https://avatars.discourse-cdn.com/v4/letter/m/a587f6/32.png) [@MikeInNH](https://community.cartalk.com/u/MikeInNH)\ **Post date:** [July 21, 2011, 1:36pm UTC](https://community.cartalk.com/t/speed-up-coast-down-to-slower-speed/66296/15 "2011-07-21T13:36:31Z")
Either he has no idea how to apply Einsteins’ formula…or it’s a joke…I think it’s a joke. ---
**Author:** ![the\_same\_mountainbike](https://avatars.discourse-cdn.com/v4/letter/t/c89c15/32.png) [@the\_same\_mountainbike](https://community.cartalk.com/u/the_same_mountainbike)\ **Post date:** [July 22, 2011, 5:25pm UTC](https://community.cartalk.com/t/speed-up-coast-down-to-slower-speed/66296/16 "2011-07-22T17:25:42Z")
Floyd’s arithmetic is correct. But he ignores drag. It’s a drag trying to maintain one’s sanity behind someone who keeps speeding up and slowing down. And for these purposes drag is directly proportional to the variation in speed. The greater the variation in speed, the greater the drag. Sorry. Couldn’t resist. ---
**Author:** ![circuitsmith](https://sea1.discourse-cdn.com/cartalk/user_avatar/community.cartalk.com/circuitsmith/32/40376_2.png) [@circuitsmith](https://community.cartalk.com/u/circuitsmith)\ **Post date:** [July 22, 2011, 5:50pm UTC](https://community.cartalk.com/t/speed-up-coast-down-to-slower-speed/66296/17 "2011-07-22T17:50:14Z")
I think Floyd meant to use e=mv2, where e=kinetic energy. Not at all the same as the equivalence between mass and energy ---
**Author:** ![the\_same\_mountainbike](https://avatars.discourse-cdn.com/v4/letter/t/c89c15/32.png) [@the\_same\_mountainbike](https://community.cartalk.com/u/the_same_mountainbike)\ **Post date:** [July 22, 2011, 8:53pm UTC](https://community.cartalk.com/t/speed-up-coast-down-to-slower-speed/66296/18 "2011-07-22T20:53:09Z")
No, I think he meant e=mc2. Floyd is used to traveling at the speed of light…squared. ---
**Author:** ![Jussi](https://avatars.discourse-cdn.com/v4/letter/j/3e96dc/32.png) [@Jussi](https://community.cartalk.com/u/Jussi)\ **Post date:** [July 23, 2011, 1:47am UTC](https://community.cartalk.com/t/speed-up-coast-down-to-slower-speed/66296/19 "2011-07-23T01:47:54Z")
I live in a hilly area. Drived 2001 Honda civic manual. Driving the same road at 80 kph average with constant speed of about 100 kph will have average mileage of 9.5 l/100km, while speeding from 90 to 120 kph up the hill and switching to neutral downhill will get about 8.5 l/100 lm. (you do the math for your screwy measure of MPG) I have not been able to repeat the benefit on flat surface. To translate these numbers to US you have to count that our as has 25 % methanol in it, whus will reduce the mileage with about 10%. Unfortunately I decided to buy a car with automatic trasmission and I cannot turn to neutral while moving without wrecking the transmission. ---
**Author:** ![bscar](https://sea1.discourse-cdn.com/cartalk/user_avatar/community.cartalk.com/bscar/32/34_2.png) [@bscar](https://community.cartalk.com/u/bscar)\ **Post date:** [July 23, 2011, 3:05am UTC](https://community.cartalk.com/t/speed-up-coast-down-to-slower-speed/66296/20 "2011-07-23T03:05:27Z")
To convert Mr. Wonky Measurementland’s numbers into the superior measurements we use here in the USA 😛 80kph = 49.71mph 100 = 62.14 90 = 55.92 120 = 74.56 9.5l/100km = 24.76mpg 8.5 = 27.67 Though, with instant fuel readouts, they aren’t always accurate. [Next page](https://community.cartalk.com/t/speed-up-coast-down-to-slower-speed/66296.md?page=2)