Shifting into Neutral to Coast

[shanewithay]

One of my frequent routes is very hilly. I have a 2000 Honda Accord V6 automatic trans. My gas mileage increases if I shift into neutral going down the hills and then at the bottom of the hills shift back into drive and ease into the pedal. I probably do that 20 times or more on trip one way. Is that bad for my engine/transmission to do that regularly?

Thanks.

[WRC9000]

I would assume so. I don't think coasting in neutral is really safe for anything but a manual. However, it sounds like your doing it so that wear/stress is minimized. I belive it will be okay. The only thing you really don't want to do is pop it into drive while fulluy stopped and revving your engine to "launch" it. This will definitley break your car.

So basically, make sure you're rev-matching, if that's at all possible in an auto, and you should be okay.

[MagicRat]

Originally Posted by shanewithay

I shift into neutral going down the hills and then at the bottom of the hills shift back into drive and ease into the pedal.

This is called "Georgia Overdrive" and has been a popular way to increase mileage (slightly) for years.

If you switch the engine off when coasting down the hill you will save even more gas. This is called "Mexican Overdrive".

Locking bumpers with the big truck in front of you, placing it in neutral and switching off the engine is called "Newfie Overdrive"






(With sincere apologies to ethnic communities everywhere no offense is intended.)

[curtis73]

It also doesn't help mileage, in fact it can make it worse on modern EFI cars.

When decelerating in gear, the engine injects NO fuel since the transmission is pushing the engine. If you put it in neutral and let it return to idle, it is injecting fuel enough to maintain idle.

The chances of damaging the transmission are very small, but keep in mind you're operating the transmission at 70 mph, but the pump is only spinning at idle.

[shanewithay]

Originally Posted by curtis73

It also doesn't help mileage, in fact it can make it worse on modern EFI cars.

When decelerating in gear, the engine injects NO fuel since the transmission is pushing the engine. If you put it in neutral and let it return to idle, it is injecting fuel enough to maintain idle.

The chances of damaging the transmission are very small, but keep in mind you're operating the transmission at 70 mph, but the pump is only spinning at idle.

That's exactly what I wanted to know. Thanks so much for your help!!!

Shayne

[KiwiBacon]

Originally Posted by curtis73

It also doesn't help mileage, in fact it can make it worse on modern EFI cars.

When decelerating in gear, the engine injects NO fuel since the transmission is pushing the engine. If you put it in neutral and let it return to idle, it is injecting fuel enough to maintain idle.

Coasting in neutral with the engine idling your car will only burn 3l or so of fuel per hour. Coasting in gear you'll be burning no fuel, but will have to put in more fuel a short time later to regain the momentum lost by using the engine as a brake.

[curtis73]

True, but downhill (as he describes "very hilly") the engine braking isn't a factor. He'll need the brakes periodically anyway. You would be right if he ended up losing speed going downhill, but even if he were losing speed downhill, the amount of fuel needed to recover should certainly be less than what he saved going downhill.

[evlnxr]

i have a Toyota Caldina GTT (4wd 2.0l Turbo Station Wagon, auto trans), and find that you actually need to drive down hills with the accelerator slightly depressed, otherwise if you have your foot right off the pedal it engine brakes and slows down. So if i shift to neutral, it shows a huge increase in fuel ecomony on the little digital readout, cruises on average at 10k/litre, and goes off the scale over 20k/litre.

[shanewithay]

So are you saying that if i coast down a hill in drive with my foot off the gas that I use less fuel that coasting down the hill in neutral?

[KiwiBacon]

Originally Posted by shanewithay

So are you saying that if i coast down a hill in drive with my foot off the gas that I use less fuel that coasting down the hill in neutral?

Yes, but your car will also be much slower at the bottom.
Each method suits a different type of hill. Or rather, suits a hill with different endings.

[curtis73]

I'm not sure what the hills are like where you guys live, but most of the hills where I live require using the brakes regardless of whether or not you're in gear. But, shane, the answer to your most recent question is yes if we're talking about EFI. If your foot is off the accelerator, the computer will only inject enough fuel to maintain idle speed (lets say, 500 rpms). If you are coasting downhill in gear, the wheels and gears are pushing the engine at (lets say) 2000 rpms, so the computer injects zero fuel. If you put it in neutral, the idle returns to 500 rpms because the computer starts injecting enough fuel to maintain 500 rpms.

Much of this depends on the car, the amount of engine braking it provides, the angle of the hill, the gearing of the transmission, but if you are maintaining speed while in gear coasting with your foot off the gas, you use less fuel than popping it into neutral.

[shanewithay]

Originally Posted by curtis73

I'm not sure what the hills are like where you guys live, but most of the hills where I live require using the brakes regardless of whether or not you're in gear. But, shane, the answer to your most recent question is yes if we're talking about EFI. If your foot is off the accelerator, the computer will only inject enough fuel to maintain idle speed (lets say, 500 rpms). If you are coasting downhill in gear, the wheels and gears are pushing the engine at (lets say) 2000 rpms, so the computer injects zero fuel. If you put it in neutral, the idle returns to 500 rpms because the computer starts injecting enough fuel to maintain 500 rpms.

Much of this depends on the car, the amount of engine braking it provides, the angle of the hill, the gearing of the transmission, but if you are maintaining speed while in gear coasting with your foot off the gas, you use less fuel than popping it into neutral.

Exactly what I wanted to know. Hopefully you have saved me some money!!! Thanks so much.

[Greenblurr93]

confermation of this can be found using a wideband o2... at idle you get a 14.7 A/F ratio.. when crusing down a hill in neutral... 14.7... when coasting down a hill in gear... AIR.. which meant the o2 saw a mixture of air containing no fuel... hense the injectors werent putting any fuel into the combustion chamber... and most hills where i live i need to use the brake to keep from speeding up while in gear (unless im in a low gear like 3rd or lower)

[KiwiBacon]

Originally Posted by curtis73

I'm not sure what the hills are like where you guys live, but most of the hills where I live require using the brakes regardless of whether or not you're in gear.

I've got a mix, probably just over half are rolling hills which can be coasted down. The remainder you need the engine braking of a low gear to take the load off the brakes.

Strangely enough though, the vehicles I see driving down those hills with smoke pouring off the brakes still make it through the tight corners at the bottom.