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How Much Fuel Does the AC REALLY Use?

Ever since air conditioning became a standard feature on cars, there has been passionate debate about its effect on fuel consumption. Is it negligible? Is it worth the discomfort? What about rolling down the window? How much fuel does AC really use in a modern car? There are so many conflicting reports and rumors that’s almost impossible to determine a real answer. Here, we'll go over the basics of AC and driving with the windows down (hello physics!), but at the end of the day you may make your decision based on your personal preference on any particular day. (For example, no matter how much money your car might save by not using AC, no one is driving through the desert without it!)

How much fuel does the AC really use?

Does using the AC use more fuel? Of course. While the quantity can change depending on the aerodynamics of the car, using the AC does use more fuel than driving with the windows closed. This might be an obvious answer, but many drivers have the AC on by default, and only turn it off or down when they realize they are shivering. And keeping the AC on at full blast 24/7 will eat up whatever fuel efficiency you might have had.

Does using the AC use more fuel than driving with the windows down? This is a bit trickier of a question. In most cases, it costs less fuel to keep the windows open when driving at lower speeds rather than using the air conditioner, but having the windows open can use more fuel when driving at higher speeds. The most interesting part of this is how much the shape of the vehicle makes that number change. In a study done in 2004 researchers pit an eight cylinder SUV against an eight cylinder sedan and found that the difference between the economy at high speeds with the windows up and down was much more dramatic in the sedan because of its reduced drag coefficient. While the study didn’t investigate cars shaped like today’s hypermiling Priuses and Volts, it can be assumed that the difference would only be more dramatic in a car with a specially designed body for better gas mileage.

What should you do? Depending on the shape of your car, you could squeeze some extra mileage from a tank of gas by keeping the windows down and the AC off when you’re traveling under about  60mph. Anything higher, and you’d be better off with turning on the AC. If you’re in a boxy SUV, you’ll be better off with the windows down all the time, but if you’re in a sleek fuel efficient car, you may want to adjust the speed you consider “high speed.”

Can you test this yourself? Absolutely! Fill up your tank and get started testing your car on your own to see what’s ideal for your ride. Use about half a tank to a quarter of a tank for your estimates, and track your mileage while focusing on one variable per test. High speed (highway driving) with the windows up and AC on, high speed with the windows down and the AC off, low speed (city driving) with the windows up/AC on and the windows down/AC off. Make sure you fill up your tank after each test so you get as accurate of a number for fuel consumption as possible. Then compare your numbers! This kind of testing can be as in-depth as you want and you'll know what works for the car you're actually driving on the roads you drive every day, rather than some generic test.

Is it worth it? Depending on the car and the climate you’re working with, it might not be worth it to you be uncomfortable to save a relatively minor amount of fuel, but it seems pretty low risk to turn to the window before the AC on moderately warm days in lower speed situations. Luckily, San Diego weather is so mild throughout the year that the cool ocean breeze hits several miles inland, which is wonderful when keeping cool without AC. This week, though, we bet you're using your car's air conditioner.


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