Air Conditioning Affects Gas Mileage
By Chad Upton | Editor
In most cars, the air conditioner’s compressor is powered by a drive belt on the engine. When the air conditioner is activated, the compressor adds resistance to your engine. That extra resistance means your engine requires more fuel to turn at the same speed.
In other words, using your air conditioner burns more gas than not using it. Modern cars have very efficient air conditioners, but this truth still stands.
On a hot summer day, you have to keep cool. Does that mean it’s more fuel efficient to drive with your windows down?
Yes and no.
The Society of Automotive Engineers performed a study that examined this question in detail. They performed wind tunnel and track experiments comparing a car and an SUV. With the windows down, the car was half as efficient at 50 mph (80 km/h) than the SUV’s at 30 mph (50 km/h). It’s clear that driving the car with windows down has a dramatic effect on fuel economy, but it affected the SUV even more, especially when a 10 mph (16 km/h) crosswind was added in the wind tunnel.
Consumer Reports found that below 40 mph (65 km/h), drivers are better off with their windows down and air conditioner off.
Jason Toews from GasBuddy.com found at speeds above 45 mph (70 km/h), “wind drag becomes an issue.” He says, “Drive at speeds over 55 mph with windows down and you’ll decrease fuel economy by up to 20 percent or greater.”
Myth Busters has also looked at this issue on a couple occasions. The first time around, their methodology was flawed, so they tried it a second time. The second time, they came to the same conclusion as Consumer Reports and GasBuddy, that windows down are more efficient than running the air conditioner at speeds less than 45-55 mph (70-90 km/h) and the drag at higher speeds makes the air conditioner the more efficient option.
Of course, results vary by vehicle.
There are many factors other than windows and air conditioning that affect fuel economy: drive train, power-train, body aerodynamics and tire rolling resistance. If you want to know exactly how it affects your car, you might try experimenting for yourself. Some cars have a driver information center that displays fuel economy, otherwise you could order an after market product, such as CarChip Pro ($75-$85), which may help you get that data. If you’re interested, some of these are explored in more detail in another post, The Ideal Vehicle Speed for Best Fuel Economy.
If it’s hot outside, you should probably start with your windows down anyway. This gives your air conditioner a bit of a break by clearing the extremely hot air out of the car first.
Do prefer windows down or the air conditioner on?
Photo: Tomás Fano (cc)