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Does Running Your Car's AC Really Burn More Gas?

27 November 2017

Many have wondered: Do you burn more gas when you run your car’s air-conditioning? The simple answer is yes, the AC consumes power. But the real question is, does it burn more gas to run the AC or to simply open the windows? That question is much more complicated and, in short, the answer is it depends.

Engaging the AC connects the compressor to the serpentine belt, which uses power to create cooling in the car. The power required to run the compressor slows the vehicle down a little and requires you to use more gas to compensate. With many car models you can even feel the car become sluggish and surge as the compressor goes on and off.

By the same token, opening the windows in a car decreases the aerodynamics of the vehicle creating more drag, which also increases fuel consumption.

The question is, which one increases fuel consumption, opening your windows or running the AC? While this can be and has been tested, the answer is not so clear cut and for the most part depends on the specific vehicle, driving conditions and weather. Vehicles come in all shapes and sizes with different size engines. Some days are windy and some roads have lots of traffic lights. All these conditions and more have an impact on fuel use.

Sitting at a stoplight in a car with the AC on is going to use more gas than sitting still with the windows open. The power consumption of the AC is more constant regardless of whether or not the vehicle is moving, compared to wind resistance, which is speed dependent. Wind resistance due to opening the windows is much more dependent on driving style, and the vehicle’s design. Other factors can also change wind resistance such as how much and how many windows are open.

If you are traveling slowly, windows up or down will make much less of a difference in fuel economy than at highway speeds. For around town, if you are not going too fast, particularly in stop-and-go traffic, you will likely find better fuel economy with the AC off and windows down.

At highway speeds, the answer is more difficult and the reality is that some vehicles will be better with the AC on while others will be better with windows open. On the highway the difference in fuel economy achieved with windows down versus AC on is minor, so no specific answer fits all vehicles and roads. If you find yourself running out of fuel and need every last drop, your best chance is to leave the windows up and AC off. That may not make for a comfortable ride but it will help get you to a gas station. If you don’t need every last mile out of your fuel, then the best approach is to use whichever is the most comfortable because the differences are very slight.

Why the confusion? For the average car, the difference in fuel economy between windows down or AC on is about half a mile per gallon or less. That means that for 10 gallons of gas, the difference you might get between windows open and AC on is about 5 miles or less depending on the vehicle. Because of this, two different people could come to opposite conclusions based on their experiences in the same or different cars.

Given the slight and questionable mileage differences, unless I am running out of gas with no gas station in sight, personally I would opt for the most comfortable option.

To contact the author of this article, email daniel.franklin@ieeeglobalspec.com


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Discussion – 1 comment

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Re: Does Running Your Car's AC Really Burn More Gas?
#1
2017-Dec-21 12:57 PM

With my car (10 gal tank, from 2007), maybe once every month or two, I regularly make a 320-mile trip (one way). An interstate highway makes up 2/3 of the trip, and the remaining third is half in the mountains. I can make the whole trip with some gas left (~1/10 to 1/8) with the AC off (windows cracked). I can't make it 3/4 of the way if I use the AC even intermittently (I turn it on only when going down hills if I can help it).

Heater is similar but not as bad.

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