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 14 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.

Tags: fuel economy

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