Keeping an eye on your fuel efficiency is smart, both for your wallet’s sake and the environment’s. If you’re looking to push that efficiency beyond looking for the most fuel-efficent new cars you can buy, you should know that running your car’s air conditioning requires energy, which comes from your engine. To provide that energy, your engine has to burn more fuel. So could you save a few miles per gallon if you just turned the AC off and rolled the windows down?
As usual with engineering questions, the answer isn’t straightforward. The trick is that cars are designed to be aerodynamic, moving through the air with as little drag as possible so that the engine doesn’t have to work as hard. Rolling the windows down breaks up that design, causing a slight increase in drag. Your engine will have to work a little harder in that case, meaning it has to burn more fuel, which is what you were trying to avoid in the first place.
Every car is a little different, and external conditions like temperature affect the answer as well, so there is no definitive conclusion. However, as a rule of thumb, it’s usually best to roll the windows down (and turn the AC off) at low speeds, but then switch to the AC on (with the windows up) at higher speeds to avoid the worst gas mileage.
The impact of AC and rolled-down windows on mileage
According to the U.S. Department of Energy, an air-conditioning system struggling against very high temperatures can drop fuel efficiency by as much as 25%, a huge amount. Similarly, SAE International, an association of aerospace and automotive engineers, calculated that using the AC could cost a car anywhere from 4% to 30% of its total fuel use, or anywhere between half a mile per gallon to nearly 9 mpg. Consumer Reports cites a narrower range of 1 to 4 mpg. The numbers get progressively worse the faster the car is moving.
The effect of rolling the windows down on fuel economy hasn’t been studied as much. Consumer Reports found the effect to be negligible, but SAE did find it to be around 3% to 9%, getting most severe around the 50 mph mark before falling back at even higher speeds. In the case of the test vehicle, this amounted to a loss of 1.5 to 3.5 mpg. All in all, this means that rolling down the windows might not always more fuel efficient than the AC, particularly at highway speeds.