Air conditioning or windows down? Which uses more fuel

Updated 23 September 2026

It depends on speed. Air conditioning burns an almost fixed amount of fuel per hour, so it weighs heavily in town; open windows spoil the aerodynamics, which only matters at higher speeds.

The short answer

In town and on country roads, opening the windows uses less fuel than air conditioning.
On the motorway, in a car, from about 100–120 km/h (60–75 mph) open windows use as much or more than the AC, especially if the AC is not working flat out.
In an SUV, which is taller and less aerodynamic, open windows stay cheaper than AC up to almost 130 km/h (80 mph).

That is what Oak Ridge National Laboratory (ORNL) measured with a Toyota Corolla and a Ford Explorer: with the AC at maximum, the Corolla used more fuel with AC than with open windows up to 75 mph; with the compressor running half the time, open windows became more costly above about 60 mph. In the Explorer, AC used more fuel until nearly 80 mph.

How much fuel air conditioning uses

With the compressor at maximum, ORNL measured about 0.17 US gal/h (0.64 L/h) extra in the Corolla and about 0.4 gal/h (1.5 L/h) in the Explorer, almost constant between 40 and 70 mph. As a percentage, that meant 22% more at 40 mph and 9% more at 70 mph in the Corolla: the same amount per hour is spread over more miles the faster you go.

According to fueleconomy.gov, in very hot weather AC can raise fuel use by more than 25%, especially on short trips. Averaged over a year, the Swiss lab EMPA estimated a 4.5% increase in town, 2.3% on country roads and 1.2% on motorways, and found that two thirds of that extra fuel could be saved by switching the AC off below 18 °C.

Estimated extra consumption in a compact petrol car (≈ 7 L/100 km, 34 mpg US)
Road type	Air conditioning (above 30 °C)	Windows down
Town (≈ 25 km/h)	+2.2 L/100 km (≈ +29%)	+0.1 L/100 km (≈ +1%)
Country road (≈ 70 km/h)	+0.8 L/100 km (≈ +13%)	+0.2 L/100 km (≈ +4%)
Motorway (≈ 110 km/h)	+0.5 L/100 km (≈ +7%)	+0.5 L/100 km (≈ +7%)

Values from the calculator’s model (ORNL data for AC and windows). At 25–30 °C the AC uses roughly 35% less.

Why open windows cost more at speed

Aerodynamic drag grows with the square of speed. With the windows open, air flows into the cabin and disrupts the flow around the car. In track tests, ORNL measured a penalty of 4–8.5% for a compact sedan and 1–4% for an SUV, which already starts from much higher drag. At low speed the effect is almost nil.

Tips to use less fuel in summer

Park in the shade or use a sunshade: the AC will have less heat to remove.
When you set off, open the windows briefly to let the hot air out, then switch the AC on.
Windows at low speed; air conditioning on the motorway (fueleconomy.gov).
Don’t idle with the AC on: without moving, all of that fuel is extra.