Using the air conditioner instead of rolling down the windows improves fuel efficiency at higher speeds, primarily over 65 miles per hour, but decreases fuel savings at speeds under 65 miles per hour. Driving with the windows down makes cars fuel efficient in urban settings, such as city driving. However, many other factors, such as vehicle type and outside temperature, also affect fuel economy.
Smaller cars, like sedans, see slight improvements in fuel efficiency by switching to air conditioning and rolling up windows at around 75 miles per hour. Driving at that speed increases drag, which makes driving with windows down less fuel-efficient than turning on the air conditioning unit. With larger cars, however, such as sports utility vehicles, little difference occurs when driving with the windows down and air conditioning on speeds exceeding 60 miles per hour. Since drag, produced by air circulating through the car, reduces fuel efficiency, driving with the windows rolled up and air conditioning unit off produces the most fuel savings. Variations in fuel economy occur from many factors, such as size of car, humidity, temperature and volume of traffic. At certain speeds, little variation exists in fuel savings between using air conditioning and driving with the windows rolled down.