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Why does coasting downhill use more fuel than staying in gear?

The leaflets sent out with tax disc renewal notices headed “Be a smarter driver and save money” give the advice “Stay in Gear. Don't coast downhill or towards lights in neutral as this uses more fuel.” While I appreciate that it is safer to keep a car in gear, I don't understand how or why coasting should use more fuel than being in gear. Can you explain?

Asked on 15 September 2012 by TC, Canterbury

Answered by Honest John
Coasting in neutral uses a tiny amount of fuel to keep the engine idling. If you lift off in gear, the fuel supply is shut off entirely, the wheels turn the engine via the drivetrain and the engine uses no fuel at all.

Real MPG enables you to register the fuel economy you are actually getting against your make and model of car, its engine and type of transmission.

Every time you enter an average consumption figure against your car, the overall average automatically adjusts to reflect it.

Real MPG
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