‘How much fuel does my car’s air conditioning use?’

Dear Alex,

Even in very hot weather, a friend never uses his Honda Civic’s air-conditioning system, in order to save fuel. Instead he drives with the windows open, which I would have thought would be less economical due to the increased drag. Which of us is right? 

– BS

Dear BS,

It’s a tricky question, as there isn’t an exact science behind working it out – although many have tried.

The general consensus is that opening windows is more fuel efficient at lower speeds (in town, for instance) but at higher speeds, say on a motorway, the amount of drag an open window creates has a greater impact on fuel economy than using the air-con.

The speed that the change-over takes place will vary according to car type and model. For example, an SUV’s boxy shape will already be high on drag, so opening the windows won’t have quite as much of an impact. However, in a smooth, sleek coupé, open windows will create far more extra drag. A good rule of thumb is to close the ­windows and switch on the air-con at about 45mph.

However, if you want to use the air-con all the time, you can make its life easier by emptying your car of hot air as you pull away. Open all four windows as soon as you get in the car, then close the front two after a few minutes’ driving, allowing warmer air to be drawn out of the back of the car while cooler air is fed in by the air-con at the front. After a few more minutes, you can close the rear windows. Doing this cools the car far more quickly than simply switching the air-con to full blast – and uses less fuel, too.

There’s also the point that your friend’s air-con may not work correctly after so many years of disuse. That might well be a false economy, because if he has to get the system working before he can sell the car, or deduct money from a part-exchange value as a result of its being defunct, the sums involved are likely to surpass any savings he has made on fuel.

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