Why the AC uses gas
A gas car's air conditioning compressor is driven by the engine. When the AC is on, the engine has to make extra power to run it, and extra power means extra fuel. That's why fueleconomy.gov calls running the AC "the main contributor to reduced fuel economy in hot weather."
The size of the hit depends on how hot it is, how hard the AC works and how you drive. The worst case is a hot day full of short trips, when the system is working flat out to cool a cabin that has been baking in the sun. That's where the "more than 25%" figure comes from. On a long highway drive with the cabin already cool, the AC's share of the engine's work is smaller, so the percentage drops.
Hybrids, plug-in hybrids and electric cars feel it too. fueleconomy.gov notes that the AC's effect on those vehicles "can be even larger on a percentage basis," because they use so little energy to begin with.
What the AC costs: a worked example
- Without AC: 1,000 miles ÷ 25 MPG = 40 gallons, or $140.00 at an example $3.50 a gallon
- With a 25% hit: 25 MPG × 0.75 = 18.75 MPG
- With AC: 1,000 ÷ 18.75 = 53.3 gallons, or $186.67
- Extra cost: $46.67 a month, or $186.67 over a four-month AC season
That's the hot, short-trip case. At a 10% hit, the same driving costs about $15.56 more a month. Plug your own numbers into the calculator above to see where you land, and use the fuel cost calculator if you want the cost of one specific drive.
AC or windows down: which uses less gas?
Open windows don't run a compressor, but they do make the car less slippery. fueleconomy.gov explains that open windows "increase aerodynamic drag," and that the effect "is quite small at low speeds but increases at highway speeds."
So the official advice is simple: roll the windows down at lower speeds and use the AC at highway speeds. You'll see specific cutoffs quoted online, often somewhere around 45 to 50 mph, but fueleconomy.gov doesn't give one number, because the crossover point depends on the car's shape and how much AC it needs.
How to use less fuel with the AC on
These tips come from fueleconomy.gov's hot weather guidance:
- Park in the shade or use a sunshade so the cabin starts cooler.
- Open the windows for a minute to let hot air out before you turn the AC on.
- Don't idle with the AC running before you drive. Turn it on once you're moving. Idling burns fuel on its own, and our idling cost calculator shows what that adds up to.
- Avoid very low temperature settings and running the AC harder than you need.
- In a plug-in hybrid or EV, pre-cool the cabin while the car is still plugged in.
The AC is one cost among several. Aggressive driving, extra weight and roof boxes can cost you more, and our guide to saving money on gas puts a dollar figure on each one.
Cedrick Reese wrote this guide to answer "should I turn off the AC to save gas?", a question most answers online stop at "yes, a little" without saying how much. The percentages and tips on this page come straight from the U.S. Department of Energy's fueleconomy.gov hot weather and driving pages, quoted rather than paraphrased where it matters, and the calculator applies them with plain arithmetic you can check against the worked example. The page is reviewed when fueleconomy.gov updates its guidance, and the gas price shown is only an example.
AC and gas questions
Does running your AC use a lot of gas?
It can. fueleconomy.gov says AC use can reduce a conventional vehicle's fuel economy by more than 25% under very hot conditions, especially on short trips. On mild days and long highway drives the effect is smaller. At 1,000 miles a month, 25 MPG and an example price of $3.50 a gallon, a 25% hit adds about $47 a month.
Is it cheaper to roll the windows down or use the AC?
At lower speeds, windows down usually uses less fuel. At highway speeds, open windows add drag, so the AC is often the better choice. That's fueleconomy.gov's advice: roll the windows down at lower speeds and use the AC at highway speeds.
Does the car heater use gas?
Very little in a gas car, because the heat comes from the engine's own waste heat. The fan, heated seats and window defrosters do use extra power, and fueleconomy.gov suggests not running seat warmers or defrosters more than you need.
Does AC use more gas on short trips?
Yes. On a short trip the AC spends most of its time pulling down the heat in a hot cabin, so it takes a bigger share of the fuel. fueleconomy.gov notes the more-than-25% hit happens particularly on short trips.
Does AC use gas when the car is idling?
Yes. An idling engine still powers the compressor, and fueleconomy.gov says idling can use a quarter to a half gallon of fuel per hour depending on engine size and AC use. It advises not idling with the AC on before you drive.
More fuel tools
Want the cost of one drive? Use the fuel cost calculator. To see what your commute costs over a month and year, try the commute cost calculator.
Not sure of your MPG? Look up your car's EPA rating on our sister site, GasMileageCalculators.com.
Sources
Last reviewed: October 4, 2026.