Running your car’s air conditioning, like any other feature on your car that draws power, will affect your gas mileage. The question is, how much will it affect the mileage, is it enough to warrant not using the air conditioning, and are the alternatives any better? The answers vary between car models, as well as between individual people because driving habits change from person to person. Good, master-certified automotive technicians should know what to consider if a customer asks whether running the air conditioner will be bad for mileage on that customer’s particular car.
Your Choices are Limited
The primary choices to consider are running the air conditioner with the windows up, or turning off the air conditioner and either using the fan-only setting, or no setting. In other words, there aren’t many.
With the air conditioning, you’re actually siphoning power away from the engine and using more fuel to keep the air conditioning going. With the windows down, you’re allowing air to enter the car and pretty much slam against the back window, creating a drag effect that requires the car’s engine to work a little harder to keep moving at the same speed.
Putting it to the Test
In 2004, the Society of Automotive Engineers took a look at the difference by testing a car in both a wind tunnel and on a desert track in Arizona. They tested a full-size sedan and an SUV, so keep in mind that these results might not hold for economy cars, minivans, and other vehicle sizes. Part of the test also included driving in a crosswind, which is something many drivers encounter.
The study showed that having the windows down was actually rather bad, especially while driving faster than about 50 to 55 miles per hour. Fuel efficiency dropped by a whopping 20 percent when the windows were open, but only by 10 percent when the air conditioner was on.
But in 2012, Consumer Reports tested a Honda Accord and found mileage dropping by about 3 miles per gallon when the air conditioner was used and when the car was going 65 miles per hour. Now, remember that this is a different class of car with a different engine from what the Society of Automotive Engineers tested, and the test conditions were likely different as well. But it still shows how there’s no one answer that will fit all drivers.
So Should I Run the AC or Roll the Windows Down?
Obviously, if turning on the air conditioner is visibly and immediately affecting the car’s operation, turn it off — you don’t want your engine overheating, for example. But assume for the moment that your car is fine and that you’re not limping your way to the nearest gas station, trying to preserve what you have left in the gas tank. If it is hot out to the point where you can’t deal with being in the car, or if you or a passenger have health problems related to or exacerbated by the heat, turn on the air conditioner. You’re not doing anyone a favor by trying to tough that out for 3 miles per gallon.
If it is cooler, though, you may want to turn off the air conditioner, especially if you’re traveling under 50 miles per hour. There will be less drag from wind at those speeds, which means that less gas will be burned.
As for the result from Consumer Reports, where going over 65 with the air conditioning on dropped gas mileage by 3 miles per gallon . . . you might still want to keep the windows up just so you don’t have wind hitting your face at that speed. It’s loud and can get rather annoying after a while, but that’s a personal preference that you have to decide for yourself!
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