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Old 08-06-2012, 11:00 AM   #1 (permalink)
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Coast in-neutral vs in-gear: I've been challenged to show the numbers

... which I thought would be easy to do by searching this forum. Well, it's turned out to be easier said than done. I've been through several threads where I expected to find the numbers to back up the logical conclusion that coasting in neutral loses less momentum and therefore results in lower fuel use. However, nobody seems to have done an A-B-A comparison, much to my surprise. The closest I've found is this post:
http://ecomodder.com/forum/showthrea...html#post41066

But he didn't do full tank comparisons of the techniques, which is what I think will be the best (only) way to convince the doubters. Has anyone done such a test and can post the results?

I know that the idea, once correctly explained, is so logical that testing it seems absurd, but some people take quite a bit of convincing.

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Old 08-06-2012, 12:17 PM   #2 (permalink)
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Why bother, it would be like convincing someone fire engines are red. Its obvious.
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Old 08-06-2012, 01:06 PM   #3 (permalink)
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It's hardly possible to get decent numbers in normal driving, as you can't even start measuring from the spot where the coasting starts.

It'd have to be measured between a spot where one enters with X mph, and a place where one has to turn off at Y mph - that'd have to be reachable by coasting.

I started coasting shortly after joining ecomodder.
Coasting (even in neutral with the engine on) is what got me consistently below 5L/100km / over 47 mpg.
Up to then, I was avidly using engine braking, and only once in 5 years got below 5L/100km / over 47 mpg.


People are easily misled by the fact that their engine doesn't use gas when decelerating and letting of the gas pedal.
They don't realize they are also going to stop far, far sooner
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Old 08-06-2012, 01:27 PM   #4 (permalink)
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I think what my challenger is after is more of a net mpg gain as measured at fuel-up time. Not the instantaneous mpg or gph during the coast. But after thinking about it, this would be hard to accomplish wouldn't it. There would need to be a test route that allows coasting and that is long enough to allow a measurable amount of gas to be pumped at the end of it.
The conditions would need to be similar for each test, which would be something like:
1. Fill the tank
2. Drive the route using in-gear coasting
3. Refill the tank and record data
4. Drive the route using in-neutral coasting
5. Repeat 3
6. Repeat 2 and 3.

Yes, I can see why nobody has done this.
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Old 08-06-2012, 02:12 PM   #5 (permalink)
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Quote:
Originally Posted by ksa8907 View Post
Why bother, it would be like convincing someone fire engines are red. Its obvious.
Fire engines are not always red, but I agree with your premise that it should be obvious.
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Old 08-06-2012, 02:18 PM   #6 (permalink)
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Let your challenger try it

Nothing more convincing than first hand experience.
I too was reluctant to try it, but it works.

Despite the overshoots, even the first tries will yield results.
It will only get better when he gets more experience with coasting - i.e. ever more appropriate starting point / speed and thus better matching the coast to the desired speed at the next waypoint.
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Old 08-06-2012, 03:14 PM   #7 (permalink)
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I did an AB test with this on 2 separate tanks it was roughly the same temperatures and traffic, and the same route the whole time. Coasting in Neutral resulted 23mpgs while coasting in gear resulted 20mpgs, even if by chance the cars do coast the same in neutral as in gear which i feel neutral coasts further your rpms drop in neutral resulting in better mpgs.
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Old 08-06-2012, 06:22 PM   #8 (permalink)
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Coasting in gear is not really coasting, since there is engine braking involved unless clutch disengaged in a manual). You can get hundreds of MPG coasting in neutral but if you are using engine braking you can engage DFCO and get infinite mileage for a shorter distance. Ideally you should be able to coast in neutral to a complete stop for the best theoretical mileage, but it just doesn't happen to most people in typical traffic.

I coast in neutral when it will not aggravate other drivers, then use engine braking when I need to slow down more rapidly.

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Old 08-06-2012, 07:56 PM   #9 (permalink)
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Thanks, but I've seen this reasoning hundreds of times in all the prior threads on the topic. And very convincing it is ... to me at least. It hasn't been until gealii's post that I've seen someone back up the reasoning with real numbers. You're all probably right that those numbers won't convince my challenger, but I do believe that you people have risen to the challenge. Thank you.
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Old 08-06-2012, 08:04 PM   #10 (permalink)
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Quote:
Originally Posted by reb01501 View Post
Thanks, but I've seen this reasoning hundreds of times in all the prior threads on the topic. And very convincing it is ... to me at least. It hasn't been until gealii's post that I've seen someone back up the reasoning with real numbers. You're all probably right that those numbers won't convince my challenger, but I do believe that you people have risen to the challenge. Thank you.
also on those numbers the car had directional snow tires on it so the amount would have been smaller then a normal cars tires. I did not put the tires on the car they just came with the car and for something I only planned on having for 7 months which is about what i had the car for i didn't see a point in replacing the tires

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