Does using air conditioning affect fuel economy?

The debate has started again as to whether using the air conditioning is more fuel-efficient than having the windows down. Which is correct?

Asked on 8 April 2012 by JY, Chewton Mendip

Answered by Honest John
Yesterday, we drove the new Yaris hybrid with windows open and got 4.6 litres/100km (61.4mpg). Today, we drove exactly the same car with windows closed and a/c on and got 3.5 litres/100km (80.7mpg) on the same flat roads in Holland.
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