Coasting downhill


#1

How bad is it for an automatic transmission to be shifted into neutral in order to coast downhill and then shifted back into drive? I’ve heard that you can save gas by doing this, but I didn’t want to blow up my car.


#2

It’s harmless, perhaps puts a bit more wear on the tranny decreasing its life by a few days (wild guess), but you’ll be really hard pressed to measure any gas savings. Your engine is at idle going down hill anyway.

The confusion comes because of the perception that since the engine is slowing the car it’s drawing gas in. In reality, the throttle position sensor is registering idle signal, and the high vacuum compared to the large volume of air coming into the snorkle (the low absolute pressure being measured by the manifold absolute pressure (MAP) sensor as compared to the high volume being measured by the mass airflow sensor (MAF)…called by some the “stochiometric ratio”, which literally translated just means “difference measurement”) is telling the computer the engine has only “idle” demand on it…

anyways, the slowing is because the engine is trying to draw air in through a passageway restricted by the now closed throttle plate.

In short, if your foot is off the pedal you’re using minimal gas whether the car is in gear or in neutral.

The good news is that you won’t blow up the car.


#3

what about with a manual transmission in gear and the engine off? I reckon the same situation applies?


#4

Do not turn your engine off. You’ll lose your power steering. You’ll still have brake boost (as long as the engine is engaged) and since the injectors will be closed and your ignition system will be deenergized you probably won’t do any damage, the engine will just be suckin’ wind, but I don’t see whay anybody would do that.


#5

Thanks for the info!