Is it really OK to shift into neutral when coasting downhill?

I drive a 92' Dodge Dynasty with an automatic 4 speed transmission. I've read in almost every "fuel saving" instuctable that if you shift into neutral while going downhill, it'll save gas. I told my dad this, excited at the prospect of getting better mileage, and he said that when in neutral, the transmission fluid doesn't circulate in the transmission and it's like the worst possible thing to do. Dad's worked on cars forever, but he learned to work on older ones. I think maybe what he's saying applies to older transmissions or something. Basically, I figure that if it really will cause damage, someone else would have spoken up by now in comments or something.

So, please, settle my nerves. I love my car and don't wanna damage it, but I also want better mileage. Is my dad right or is it safe to shift into neutral?

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Burf7 years ago
Not to contradict your dad, but to the best of my knowledge, coasting with an automatic transmission in neutral won't cause any harm, however, chances are pretty good it won't save you any gas either.
If you have electronic fuel injection and 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. But if you put it in neutral, the idle returns to 500 rpms because the computer starts injecting enough fuel to maintain 500 rpms.
So if you coast, foot off the accelerator, transmission in gear, no fuel is being injected. In neutral, the EFI injects enough gas to maintain idle speed.

CurtisR18 Burf1 year ago

I can tell you from experience, that in my 2014 ford Focus with a 6 speed auto shifting manual it makes a significant improvement in fuel economy shifting into neutral coasting downhill.

and no, I don't shut the engine off, that would be dangerous and add more wear.

Funk_D (author)  Burf7 years ago
That makes perfect sense. Funny thing is I kinda was thinking that in the back of my head when I read those fuel saving tips, but I assumed that they knew something that I didn't since I'm not a pro on cars or anything. Well, not exactly what you said but I basically wondered how it saved gas at all since I didn't have my foot on the gas. Maybe it meant that most people will step on the gas to speed up down small hills because their engines slow them down. Either way, thanks!
grizzly666 Burf7 years ago
Burf is right on modern fuelinjection systems coasting will require more fuel. Hovewer if you have an older fuel injection system that do NOT shut down the fuel when the engine does not need any fuel or have an carburetor then you will save fuel by coasting. The lubrication of an automatic transmission is done with an oil pump that is driven by the engine so if the engine is turned off the transmission DOES NOT GET ANY LUBRICATION AT ALL so it is a bad ide to coast down a very long down hill with the engine turned off. Even worse is it to tow a automatic car. Sure the trans will probably hold for a tow or four but you did take many,many miles from the life expectancy of the trans (it will fail much erlier then if you did not tow the car). The correct way to tow ana automatic car is to remove the driveshaft (so that the trans will not spinn) or use a tow truck and lift the drive wheels off the ground. Manual transmissions do not have the same weakness. They are lubricated by the gears that splash the oil around and since they do spinn when towing,coast the trans do get lubrication.
coastin'downhill in neutral is known (among other titles) as "hilbilly overdrive" But this applies to a manual xmission and also includes shuttin' off th' engine ! Its illeagle in most states 'cause is freakin' dangerous! What Burf sez is correct !
caarntedd7 years ago
With your car in neutral you don't have any drag from the engine, which you need going down hills so you don't cook your brakes.
Changing down gears (auto or manual) when going down steep hills stops the vehicle from moving too quickly, and stops you from dragging the brakes to stay at a safe speed. If the braking system overheats and the brakes start to fade, ( need to be pushed harder and harder to maintain braking capability) you will soon have no brakes while rolling down a hill in 'angel' gear.

I must be way out of touch with modern vehicles, because I've been taught ( advanced driving of cars, heavy vehicles, heavy off road vehicles) that coasting in neutral at any time is extremely dangerous. Burf is correct in saying that fuel economy is achieved in modern cars by the car's own management systems.

You may save fuel, but it may cost a life.

seandogue7 years ago
IMO/IME, Best thing would be to hold the clutch in tight and keep yourself in a gear that's suitable for slowing the vehicle if needed . Brakes are something to consider as well. They may not require gasoline, but they are expendable.