Quote:
Originally Posted by scubascooby
Most post 1980's cars shut off the injectors when the throttle is closed above a certain rpm, only starting them again to stop the engine stalling.
Coasting will burn more fuel than leaving it in gear.
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I agree with dcb.
On a car with closed throttle fuel shutoff, coasting will burn more fuel than staying in gear but will allow you to go farther before needing to accelerate (and burn fuel) again. If you need to have some braking anyway, keeping the transmission in gear is best. If not, coasting is more efficient overall. If you know you will gain speed while going down a hill, you may be able to start coasting early near the top of the hill to lose some speed, then gain some speed while descending the hill without exceeding the speed limit or the highest reasonable and safe speed. On hills that are both long and steep, it won't be useful to coast the whole way down, since the car would go too fast. The strategy to save the most fuel would be to coast early at the beginning of the hill, slow down (because the beginning of the hill is not steep enough to maintain speed), gain speed when the hill gets steeper, then shift back into gear when the speed is near the speed limit. On cars that don't shift smoothly from neutral to drive (such as some Mercedes), it would be best not to shift to neutral so the car is not at risk of skidding when engine braking is needed again. Speeding allows a longer coast after the hill but increases the risk of wasting money on tickets or having an accident. If we had regenerative braking we would be both safe and efficient in hilly areas without having to slow down a lot before going down hills.
Cars with a lot of engine braking will get the most benefit from coasting. Modern cars with overdrive transmissions usually don't have as much engine braking as older cars because of lower engine speed. Cars with low wind resistance and rolling resistance also get more benefit since they can coast a longer time before slowing down too much.