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There has been much debate about whether it is more fuel efficient to use the air conditioner or lower the windows on a hot car. With the air conditioner running on 50% duty cycle, more fuel was used on a 2009 Toyota Corolla up to 60 mph, but at higher speeds, having the windows down uses more fuel due to increased drag. On a 2009 Ford Explorer SUV, which has different aerodynamics than the Corolla, the air conditioner at 50% duty cycle used more fuel until speeds reached nearly 80 mph. Using the air conditioner at 100% duty cycle in the SUV resulted in greater fuel consumption than having the windows down at all tested speeds.
Fuel Use for Air Conditioner Use versus Windows Down for a 2009 Toyota Corolla and Ford Explorer
Note: Tests were performed on a 2009 Toyota Corolla and a 2009 Ford Explorer with three scenarios: 1) all four windows down, 2) all four windows up and air conditioner on 100% duty cycle, and 3) all four windows up and air conditioner off. Results were then modeled for a fourth scenario of the air conditioner at 50% duty cycle. Both vehicles were gasoline and were tested at steady-state speeds.
Source: Huff, S., West, B., and Thomas, J., "Effects of Air Conditioner Use on Real-World Fuel Economy," SAE International Journal of Passenger Cars Mech. Syst., 2013, doi: 10.4271/2013-01-0551.