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How do the windows and air conditioning affect fuel economy?

Which has the least effect on fuel consumption: having the air conditioning on or opening a window?

Asked on 12 October 2013 by TR, Sewards End, Essex

Answered by Honest John
Below 30mph, a/c off and windows open uses less fuel. Over 30mph, a/c on and windows closed uses less fuel.

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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