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If you're driving downhill would it be more economical to put the car in neutral and coast, or leave the car in gear and let the wheels drive the engine?

Please ignore any safety concerns for this question, and assume the car is a modern, fuel injected vehicle.

In my view coasting would mean fuel is needed to keep the engine idling, whereas leaving it in gear would mean the engine is mostly prevented by stalling via momentum rather than fuel.

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You are correct - in a modern fuel-injected vehicle, the ECU will cut the fuel right back (or even off completely) if you are coasting downhill in gear, wheras more fuel is needed to maintain an idle.

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    However, leaving the vehicle in gear will also provide braking via internal engine friction. If this braking is sufficient to reduce your speed below the desired speed, you'll have to open the throttle to maintain speed, thereby consuming fuel. So it's not quite so clear-cut. The best possible option, from a pure efficiency standpoint, is to coast in neutral with the ignition cut off (engine not turning at all). In a manual, you can restart the engine at the bottom of the hill by releasing the clutch in gear, without using electricity for the starter motor.
    – R.. GitHub STOP HELPING ICE
    Commented Sep 2, 2013 at 16:25
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    Thanks Nick C, have you any sources to back up your answer?
    – SilverlightFox
    Commented Sep 3, 2013 at 7:52
  • @R. My own observations and experiment support the notion that at least on some cars fuel will actually get injected when going downhill if you start slowing down too much.
    – Michael
    Commented Apr 6, 2018 at 18:25

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