Your air conditioner does not use gasoline directly, but it does reduce fuel economy by making your engine work harder

When you turn on the air conditioning in your car, the compressor — a pump driven by your engine — pressurizes refrigerant to cool the air. That compressor is powered by a belt connected to your engine, so running it requires your engine to produce extra power. The engine burns more fuel to generate that power, which is why your gas mileage drops when the AC is on.

The amount of fuel you use depends on how hard your AC is working. On a mild day with the AC set to a low cooling level, the difference is small — often 3 to 5 percent worse fuel economy. On a hot day with the AC running at full blast, you may see 10 to 25 percent worse fuel economy, depending on your vehicle and driving conditions. Highway driving shows a smaller percentage loss than city driving because the engine is already working at a steady rate.

Key Takeaways

  • Your car's air conditioner compressor is powered by the engine, so running AC increases fuel consumption by forcing the engine to work harder.
  • Fuel economy loss from AC use typically ranges from 3 to 25 percent depending on how cold you set it and whether you are driving on the highway or in the city.
  • Modern cars use less fuel when the AC is on than older vehicles did, because engines are more efficient and AC systems are better designed.
  • Turning off the AC and opening windows saves fuel at low speeds but costs more fuel at highway speeds due to increased wind resistance.

How the AC compressor draws power from your engine

Your engine has a serpentine belt (or sometimes multiple belts) that connects to several components, including the alternator, power steering pump, and air conditioning compressor. When you engage the AC, a clutch on the compressor engages, and the belt begins turning the compressor shaft. The compressor then pressurizes refrigerant, which circulates through the condenser, evaporator, and other parts of the cooling system.

This work requires energy. Your engine must burn fuel to produce the extra horsepower needed to turn the compressor against the pressure inside it. The harder the compressor works — which happens when you set the temperature lower or when the outside air is hotter — the more fuel your engine burns. On very hot days, the compressor may cycle on and off more frequently to maintain your desired temperature, which also increases overall fuel use.

Why fuel economy loss varies by driving conditions

On the highway, your engine is already running at a steady, efficient rate. Adding AC load increases fuel consumption, but the percentage loss is usually smaller because the engine is working at an optimal point. A car that gets 30 miles per gallon on the highway might drop to 27 or 28 mpg with AC on — a loss of about 7 to 10 percent.

In city driving, your engine is constantly accelerating and braking, so it is less efficient to begin with. Adding AC load on top of that inefficiency creates a larger percentage loss. A car that gets 20 miles per gallon in the city might drop to 17 or 18 mpg with AC on — a loss of 10 to 15 percent. On extremely hot days, especially in stop-and-go traffic, the loss can reach 20 to 25 percent because the compressor is working at maximum capacity while the engine is already stressed.

Comparing AC use to rolling down windows at different speeds

Many drivers wonder whether it is more fuel-efficient to turn off the AC and open the windows instead. The answer depends on your speed. At low speeds — under 45 miles per hour — opening windows costs less fuel than running the AC because the aerodynamic drag from open windows is minimal. In city driving or stop-and-go traffic, you will usually save fuel by turning off the AC and opening windows.

At highway speeds above 50 miles per hour, open windows create significant wind resistance that forces your engine to work harder to maintain speed. The fuel cost of overcoming that drag often exceeds the fuel cost of running the AC. On the highway, keeping windows closed and AC on is typically more efficient than rolling down windows. The crossover point varies by vehicle shape and size, but most cars reach it somewhere between 45 and 55 miles per hour.

How modern AC systems are more efficient than older ones

Cars built in the last 10 to 15 years use less fuel when the AC is on than older vehicles did, for several reasons. Modern engines are more efficient overall, so the extra load from the compressor represents a smaller percentage increase in fuel burn. AC compressors are also better engineered — they use variable displacement, meaning they adjust their output based on cooling demand rather than running at full capacity all the time.

Newer refrigerants are also more efficient than the older R-12 and R-134a used in many 1990s and 2000s vehicles. Some newer cars use R-1234yf, which requires less compressor work to achieve the same cooling. Additionally, modern cars often have better insulation and reflective windshields that reduce the heat load inside the cabin, so the AC does not have to work as hard to cool the car down.

What happens to fuel economy when you use recirculate mode

Most car AC systems have a recirculate button that closes the fresh air intake and cools air that is already inside the cabin instead of pulling in hot outside air. Using recirculate mode reduces the cooling load on the compressor because the air inside the car is already cooler than the outside air. This means the compressor does not have to work as hard, and your fuel economy loss is smaller.

The tradeoff is that recirculate mode can cause the windows to fog up if humidity inside the car builds up, and it does not bring in fresh outside air. Most drivers use recirculate mode on very hot days to cool the car down quickly, then switch back to fresh air mode once the cabin is cool. Switching to recirculate mode for short periods on extremely hot days can reduce fuel consumption compared to running fresh air mode at full blast.

Frequently Asked Questions

Does idling with the AC on use more gas than driving with it on?

Yes. When you are idling, your engine is producing just enough power to keep running, so the AC compressor load is a larger percentage of total engine output. At idle, the AC can increase fuel consumption by 20 to 30 percent. While driving, the same AC load represents a smaller percentage increase because the engine is already producing more power for propulsion.

Will turning off the AC improve my gas mileage noticeably?

It depends on your driving. In city traffic on a hot day, turning off the AC might improve your mileage by 10 to 15 percent. On the highway, the improvement is usually 5 to 10 percent. If you are already getting poor fuel economy, AC use is one factor but not usually the main one — tire pressure, driving habits, and engine condition matter more.

Does using the AC drain the battery in an electric car?

Yes, but differently than in a gas car. Electric car AC compressors are powered by the battery instead of the engine, so running AC reduces your driving range. The range loss is typically 5 to 20 percent depending on temperature and AC settings. On very cold days, heating uses even more battery power than cooling does.

Can a broken AC compressor affect my fuel economy?

A compressor that is stuck or not engaging properly usually does not affect fuel economy because it is not drawing power from the engine. However, a compressor that is partially engaged or has internal damage may draw power inconsistently, which could cause rough idle or slight mileage loss. A mechanic can diagnose this by checking whether the compressor clutch is engaging and disengaging normally.