It's something you've probably argued about with your spouse or other passengers on long car trips: which uses more gas, cooling the car by rolling down the windows or running the AC? If you argued for the former, you'd be right, according to many sources. The difference in extra fuel consumption between the two methods varies by the type of car and the speed of travel. Paying attention to these factors can help you minimize gas consumption using either cooling method.
Windows Can Be a Drag
Rolling down the window reduces gas mileage because it increases wind resistance or drag. Your car's drag is the amount of resistance it has to the air moving across it as you drive. Cars that are sleek and low to the ground, (like sports cars or electric vehicles) have comparatively less drag, which contributes to better gas mileage. Large, tall cars and trucks with boxy shapes have much more drag, contributing to relatively low gas mileage.
Driving with the windows down does increase the drag of a vehicle, but this is relative to the car's design and is affected by the car's speed. Simply put, a large, tall SUV pushes a lot of air while it is moving. Open windows on this vehicle may add relatively little to the significant overall drag. By contrast, a low, sleek roadster is designed for much less drag, and opening the windows can increase the overall drag in greater proportion.
Speed is another factor, as the drag from open windows increases with higher speed. This is true of any car, and opening your windows uses more gas at highway speeds than it does when driving around town.
The Results are In: AC is Out
In an often-cited study by the Society of Automotive Engineers (SAE), researchers compared the fuel efficiency of two large vehicles—an SUV with an 8.1 liter, 8-cylinder engine, and a sedan with a 4.6 liter, 8-cylinder engine—at low, medium and high speeds. Both vehicles got the best gas mileage when their AC was off and the windows were rolled up (no surprise there). When the windows were rolled down, fuel efficiency dropped, especially for the sedan with lower drag. It didn't affect the SUV as much because the SUV already had a lot of air drag. When the air-conditioning was turned on and the windows rolled up, fuel efficiency was at its worst.
Researchers at Edmunds.com found similar results when testing a pickup truck, which got almost 10 percent better gas mileage with the windows down and the AC off when driving at 65 mph. Edmunds noted, "the air-conditioner produced a measurable drain on the engine and a resulting drop in fuel economy ... [perhaps because] the aerodynamic qualities of trucks are more similar to a brick wall than a sedan."
The U.S. Department of Energy reports that:
AC use can reduce a conventional vehicle's fuel economy by more than 25 percent, particularly on short trips.
- Use AC more often at highway speeds and less often during city driving or in stop-and-go traffic.
- Set the AC as high as your comfort level allows.
- Air out the car by rolling down the windows before turning on the AC and rolling up. Releasing hot air from the cabin means the AC doesn't have to cool that air.
- Start the AC only after the car is moving. Don't run the AC while the car is idling in the heat.
- Follow the recommendations in the owners manual for operating the AC. Many cars include a recirculating fan to speed cooling of a hot car. Typically, you turn this on for a couple of minutes before switching back to fresh airflow.