A/C vs Windows Down: Which Actually Uses More Fuel?
Updated 2026-06-16
Air conditioning vs open windows: the answer depends on how fast you're going.
Air conditioning does cost fuel
The A/C compressor is driven by the engine, so running it uses fuel — generally a few percent, and more in stop-and-go heat or at idle, where there's no airflow and the compressor is a bigger share of the load. This is real but usually modest on a modern car.
Windows down isn't free either
Open windows add aerodynamic drag, and drag grows with speed. At low city speeds, windows-down is cheap and beats running the A/C. At highway speed, the added drag from open windows can cost as much as — or more than — the A/C would.
So the rough rule: windows down around town, A/C on the highway. The difference either way is small for most cars; comfort and safety matter more than chasing a fraction of an MPG.
Don't overthink it
Studies generally find both effects are small in normal driving. Use whichever keeps you comfortable and alert. If you want a bigger fuel win, your speed, tire pressure and the vehicle itself matter far more than the A/C button — see our habits guide.
FAQ
Is it cheaper to run A/C or roll the windows down?
Windows down is usually cheaper in the city; A/C is usually cheaper at highway speed because open windows add a lot of drag fast. Both effects are small.
Does A/C use more gas at idle?
Relatively, yes — at idle there's no airflow and the compressor is a bigger share of the load, so the percentage hit is larger than when cruising.
Sources: U.S. DOE — driving more efficiently · U.S. EPA / DOE FuelEconomy.gov