Should I coast downhills to save fuel or leave the car in gear?

With my road tax renewal letter came a DoT leaflet on petrol saving. I was surprised by one of the tips: "Don't coast downhill or towards lights in neutral as this uses more fuel. Stay in gear and ease off the gas gently to reduce fuel flow to the engine to virtually zero." This reads wrong to me. What do you think?

Asked on 6 September 2012 by FH, London SE10

Answered by Honest John
It's correct. There's not a lot of difference, but knocking the car out of gear means the engine has to idle and uses a small amount of fuel to do so. Lifting off, the fuel supply to the engine is shut off completely and the engine is turned by the drivetrain.
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