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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.

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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