I know that it鈥檚 bad for fuel efficiency and the environment if I turn on my car鈥檚 air conditioning on a summer鈥檚 day. But surely it is also bad to drive with the windows open because as my speed increases so does the effective ventilation and the fuel cost caused by added aerodynamic drag. So at what speed is it most environmentally friendly to simply roll up the windows and turn on the air conditioning?
This is one of those questions that looks as if there is no straightforward answer, simply because so many factors are involved- Ed
鈥� There have been a number of studies on the efficiency of cars in relation to open windows and air conditioning. In 1986, Cecil Adams of the newspaper column 鈥淭he Straight Dope鈥� attempted to test this (). Driving a four-door Pontiac 6000LE he drove 500 kilometres at an average speed of 96 kilometres per hour (60 miles per hour).
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His results showed that he could achieve 12.4 kilometres per litre (35 miles per US gallon) with the air conditioning off and the windows up, 12.2 km/l with the windows up and air conditioning on, and 12.0 km/l with four windows down and the air conditioning off, showing it to be more fuel efficient to have the air conditioning on and the windows up. This study was conducted in Ohio in May.
An investigation by the Florida Solar Energy Center reached a different conclusion. Conducted with a Volkswagen GTI, this test found that at 108 km/h the increase in consumption was 3 per cent with the windows down, whereas with the air conditioning on it was 12 per cent. This study was conducted in July in Florida.
I鈥檒l leave it to somebody else to sort out if the month, geography and climate are effecting the results.
David Liptrot, Reading, Berkshire, UK
鈥� It will vary from car to car, depending on how aerodynamic the car is and how heavy it is.
Another factor to consider is the difference between the ambient temperature and the temperature selected on the air-conditioning controls. In short, it is not possible to quote one speed that is accurate for all cars.
Peter Sharpe, Murwillumbah, New South Wales, Australia
鈥� The American TV show Mythbusters tested this using identical cars with the same amount of fuel. It found that at speeds up to 80 km/h it was more efficient to leave the windows open, and at higher speeds it was better to use air conditioning.
Hoff Wendell, By email, no address supplied
鈥� A study in 2004 for the US Society of Automobile Engineers shows just how difficult this question is to answer (). It looked at the impact of air conditioning on the fuel efficiency of a large family car and an SUV. Tests carried out at General Motors鈥� desert proving ground in Mesa, Arizona, showed that at speeds above 56 km/h winding down the windows is preferable to switching the air conditioning on (see Graphs).
The results need to be interpreted with caution however, as they were sensitive to a variety of factors including external wind speed and direction and ambient temperature. Unsurprisingly, fuel economy at high speed is much better in more streamlined cars.
Lionel Cooper, Nambucca Heads, New South Wales, Australia