How Extreme Temperatures Quietly Wreck Your Gas Mileage

If you've noticed your fuel economy slipping during the coldest winter days and the hottest summer afternoons, you're not imagining it. Extreme temperatures take a real toll on gas mileage, but the reasons differ depending on the season.

Cold weather is the biggest culprit. According to the EPA, gas mileage at 20 degrees Fahrenheit drops by 15% compared to a comfortable 77 degrees. On short winter trips of just three or four miles, that drop can reach 24%. Hybrids fare even worse, with mileage falling 30 to 40% in cold conditions, and EVs lose even more than hybrids.

The cold hurts efficiency in several ways. Oil and other fluids thicken, increasing friction between engine parts. Batteries weaken, putting extra strain on the alternator. Running the heater, defroster, and heated seats also consumes power. Cold air is denser, forcing the car to push harder through it, and snow or icy roads add resistance.

Tire pressure drops in cold snaps, reducing traction and making the wheels work more. Idling to warm up the car burns gas too. Short trips are especially hard because the engine never reaches its most efficient operating temperature.

Summer heat, on the other hand, can actually improve gas mileage, warm air is less dense, so there's less wind resistance, and the engine warms up faster. Summer-blend gasoline is also more efficient than winter blends. But the catch is air conditioning. Heavy A/C use can cut fuel economy by more than 25%, according to the EPA.

The impact is greater on more efficient drivetrains and on short trips. 5%. Rolling down the windows instead of using A/C helps at lower speeds, but a 2013 study using a Toyota Corolla and Ford Explorer found that above 75 mph, the Corolla lost just as much efficiency with the windows down.

Light-colored cars may help, since they reflect heat and stay cooler inside.