Entering a car on a hot day is so unpleasant. The air is humid, the steering wheel is scorching hot, and the seat feels sticky. Understandably, there’s an urgency to turn on the ignition and blast the car air conditioner but hold on for a moment. If you want to save fuel and money, don’t rush to turn the AC on. After all, many people go on road trips or travel to holiday spots. With the extra driving, many people want to be as cost-efficient as possible.
Which is Better: Car Air Conditioner or The Windows Down?
It’s no secret that the car air conditioner uses more fuel. However, opening the windows isn’t the perfect alternative. When the car windows are down, it increases drag, slowing the vehicle — which means more fuel is needed to run. Imagine the fully open car like a parachute, slowing it down. This is especially the case driving at high speeds, like on the highway.
So researchers from General Motors and SAE (which sets standards for multiple engineering industries) tested the two cooling options to see which was more economical. They tested a sedan and an SUV in a wind tunnel and a lap track in 86 degree F weather. They alternated driving at speeds of 31 mph, 50 mph, and 68 mph.
In the end, they discovered that using the AC needs more gallons of gas per mile than keeping the windows down. Of course, the most economical option was to keep the windows up, keep the AC off, and sweat like a pig. However, most people who are fuel-conscious would prefer to just roll the windows down. 
Keep in mind that the hot weather alone can actually improve your fuel economy. The weather naturally heats up the engine to an efficient temperature more quickly. Plus, the warm air causes less aerodynamic drag than the cold. Also, summer grades of gasoline may have a little more energy for the vehicle. It’s the matter of the car air conditioner or keeping the windows open that may increase costs.
However, according to David Bennett, manager of repair systems for the American Automobile Association (AAA), the added costs of a car air conditioner don’t really make a difference. “The AC system, when operating, does add a slight load to the engine, which could slightly increase gas usage,” he said. “This usage would not be recognized by most consumers.”
As far as lowering the windows instead of using the AC, he explained, “At a very high level, there may be slight gas savings with the AC off and windows up. However, the savings would be minor and not realized by most consumers.” 
How to improve your fuel economy in the summer:
- To cool off at slower speeds, lower the windows, but use the car air conditioner at high speeds.
- Don’t set the AC at a lower temperature than you actually need. Also, turn off the AC when it’s no longer needed.
- Use a sunshade or park in a shady area to avoid the car becoming too hot.
- In the beginning of a drive, lower the windows to let out the hot air and have the car cool faster. Then close the windows and turn on the AC.
- Most car air conditioners work best while driving, so don’t turn on the AC until you start moving.
- Check the car manual for tips on how to best use and maintain the AC system. 
- Ensure your tires are properly inflated. Underinflated tires cause more rolling resistance, which requires more fuel to compensate.
- Avoid hard braking and rapid acceleration, which uses more fuel.
- Avoid idling too much. If you are stopped in your car for more than a minute, cut the engine.
- Keep up with car’s maintenance according to the manufacturer’s recommendations.
A Little Bit of Both?
According to Christopher Neigher from How Stuff Works, using a combination of the car air conditioner and rolling the windows down is the best bet.  For instance, you could open the windows to let out the hot air at the start of the drive, and switch to the AC once you start driving at higher speeds. Or if you’re cruising and enjoying the view, lower the windows for fresh air and less aerodynamic drag.
Of course, on crazy hot days, there’s nothing like blasting the AC to cool down. Sometimes the increase in needed fuel is not steep and very much worth the cool air.
- “CHARTS: It’s More Efficient To Drive With Your Windows Open Than Air Conditioner On.” Business Insider. Alex Davies. September 27, 2013
- “Fact or Fiction: Does Driving Without AC Actually Save Gas?” Family Handyman. Gael Fashingbauer Cooper. March 11, 2021
- “Fuel Economy in Hot Weather.” fueleconomy.gov. U.S. Department of Energy.
- “Windows down or A/C on — which is more fuel-efficient?” How Stuff Works. Christopher Neigher.