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Old 07-04-2008, 01:05 PM   #1 (permalink)
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Coasting in Neutral Vs. Coasting in Drive observation

For the past 3 days I have been using attempting to use hypermiling techniques to save gas on my rental car. I have been using only 3 techniques really, coasting in neutral as much as reasonably possible, keeping revs below 2 grand, and minimizing brake use.

However, I read on this forum and other places around the interweb that coasting in neutral may not save you gas as opposed to coasting in drive due to the fact that modern ECUs on EFI cars will utilize a fuel cut, if the throttle is detected as being closed. (doesnt actually have to be closed but thats a different topic)

So I decided to test this in my wife's 2002.5 Jetta GLX. It has a 2.8l v6, and a gas milage readout for both instantaneous gas mileage and average trip gas mileage.

I drove a short trip on a hilly road in my area, about 11 miles round trip.
When coasting in Neutral at 60mph I obtained 180 mpg and that slowly dropped as my speed dropped all the way down to 71mpg by the time I hit 30mph.
When coasting in Drive at 60mph I obtained "--.-"mpg on the readout. As coasted down the readout jumped to 199mpg at 50mph, and quickly dropped down to 160mpg at 49mph. From there it matched the Neutral 71mpg readout at 30mph when I put it back in drive.

notes:
both "coasts" were performed on the same hill, from the same starting point.
The Neutral coast went DRAMATICALLY further. I believe this engine when in gear causes an unusual amount of engine braking. Unfortunately I cant explore this further until I purchase a VAG-COM to really dive into the fuel and timing while coasting in drive.
the "--.-" readout is the ecu indicating the fuel cut in drive. However after repeating this I realize that the ECU will only cut the fuel above 50mph.

my conclusion:
Im going to continue to coast in neutral.
While there is a definate fuel cut in drive, I can coast significately futher with noticably less resistance when in neutral.
Also since the fuel cut only occurs above 50mph, it doesnt even make sense to coast in drive on surface streets below highway speeds. I am pretty sure on average coasting in neutral will save me more gas.
This also negates entirely the use of cruise contral, since it does not allow the use of coasting at all.

and this is my first post, yay!

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Old 07-04-2008, 03:09 PM   #2 (permalink)
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Thanks for the numerical data on this. This will be one of the things I will test to see the impact for my automatic xA on flat roads (drive coast vs neutral-on coast) when the SGII comes in next week.
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Old 07-04-2008, 04:35 PM   #3 (permalink)
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I think people should test this on each of their cars as it is hard to make generalizations with so many different type of transmissions and engine control systems.

In general, a manual car will stop dramatically quicker in gear, whereas some autos will stop quicker and some autos won't. The automatic Aristo transmission for example simply freewheels when you let off the gas. That is, until you put the transmission in L or 2 or remove o/d... then it will behave more like a manual and provide engine braking.

Also, some cars keep injecting fuel when you let off gas so that it is just like being neutral, some keep it cut as long as the vehicle has some movement, others above 20km/h, etc.. It's all very manufacturer, and vehicle dependant.

I find that I don't have room to neutral coast with traffic, but I can usually engine brake and increase stopping distance enough for me, but not enough to really anger the guy behind me. On the GT fuel stays cut until you're just about stopped.
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Old 07-05-2008, 10:48 PM   #4 (permalink)
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Yeah thats why I wanted to test it on my car. On my NX2000, I used a consult emulator to view the engine parameters. That car did not cut fuel at all. It would only cut timing to keep the engine from dying, and use this secondary air injection system to burn up the unused fuel from braking so it wouldnt burn up the cat. Very inefficient system.

Neutral is, well was, my friend in that car. rated for 29mpg highway, my best was 37.5mpg
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Old 07-06-2008, 11:09 PM   #5 (permalink)
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fonque: Sounds like I'm using some of the same techniques you are. I don't coast in neutral a lot, but I try to remember to do it when exiting the freeway and on a few surface streets where I have a long, clear view to the next intersection. I think it has helped me get the 44% over EPA on my last tank of gas, but it would be more clear of course with instrumentation. We'll see over time if how my MPG holds up. But I plan on trying to do more coasting in neutral when I feel it's safe.

At a few freeway exits I use regularly, I can actually start the coasting a bit before the exit (as long as no one is right on my bumper), coast all the way up the off-ramp, and just tap my brakes a bit at the cross-street. I don't seem to go that far during engine braking, so I almost certainly would use more gas to engine brake then power up the remainder of the ramp.
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Old 07-07-2008, 08:21 AM   #6 (permalink)
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Hello,

The question is: how long do you need to coast? Is it better to coast for 1/2 mile to 1 mile (or more!) in neutral (yielding 130-325mpg in my xA, depending mostly on how fast I'm going); or to coast for a few hundred feet/yards (netting virtually infinite mpg)?
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Old 07-07-2008, 12:26 PM   #7 (permalink)
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Mine provides quite a bit of engine braking when idling in gear. After about 5 seconds, it cuts off the fuel and the engine brakes harder. I lose speed idling down a 6% grade. Good car for the mountains though. Idling in gear at 55 Im at 1500 rpm, same as driving at 45 with the TCC locked. In neutral it runs at 1000 rpm.

I can go quite a long ways in neutral. But Ive only done it sometimes when Im approaching a stop sign, or sharp corner of 25 mph or less. Then it re-engages seamlessly. Otherwise, even when matching revs, I dont like how the trans feels when re-engaging. Saving a little gas isnt worth it if I kill the trans.

So whether its worth it or not to go to neutral to go a few hundred yards, Id say it depends how the trans acts when re-engaging. If it takes like 3-5 seconds for it to decide to get into the right gear and then lockup the TCC, Id think youd waste more gas doing that than having done nothing.
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Old 07-07-2008, 11:08 PM   #8 (permalink)
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right now, im coasting in neutral down long hills, I do not coast on the highway unless there is heavy traffic, because I stay in one lane. I also coast in neutral to stoplights, especially if I catch them early.
Coasting in neutral to a stop seems to make more sense to me than engine braking.
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Old 07-10-2008, 08:06 PM   #9 (permalink)
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engine braking, or coasting in gear, or lifting your foot off the gas, is just like "braking" w/o the use of the brake pedal. in neutral, you somehow conserve your momentum, that's why you reach farther in contrast to coasting in gear. the only force that is allowing you to decelerate in neutral is friction, gravity, road curvature, and the air. this is assuming that you are the only vehicle on the road.

some good flat pavements even when curved actually prolong the effect of coasting in neutral. based on my observation, wide curving roads that are banked actually prolong the coast when entered properly (right speed).
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Old 07-10-2008, 11:06 PM   #10 (permalink)
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Hello,

Using the engine to brake is better than using the brakes, because it (usually) shuts the fuel off completely. Actually as long as you have downshifted, you can also use the brakes, and still gain the zero fuel consumption.

Coasting in neutral is best when you can carry the momentum for a long distance. Using the engine to brake is best when you need to stop, or slow down more quickly than coasting in neutral.

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