Does air-conditioning wreck my car’s mpg?

Honest John advises a reader that’s unsure of the effect of air-con on their car’s mpg.

I have always understood that running air-conditioning causes an increase in fuel consumption. Last week, in very cold weather, I was taken for a demonstration drive in a new Toyota Avensis. The dealer kept the air-con on all the time. When I queried this, I was told it makes hardly any difference to mpg figures. Is that right?
…JM, Prestbury

Yes. In summer the air-con uses more fuel than opening the windows up to about 40mph. Beyond that, the extra drag caused by opening the windows uses more fuel than the air-con.

In the situation you describe, the air-con probably needed to be on to keep the windows clear. If a car has air-con, it’s best run for 10 minutes every week through the winter in order to lubricate the system’s seals and keep them gas-tight.

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