Hot Vs Cold: How Temps Affect Your Car's Gas Mileage

Have you noticed your gas mileage going down in the dead of winter and the hottest days of summer? You're not imagining things. Fuel efficiency goes down in extreme temperatures, though not always for the reasons you may think.

Extreme cold temperatures are the absolute worst for gas mileage. According to the EPA, gas mileage at 20 degrees Fahrenheit is 15% lower than gas mileage at a comfortable 77 degrees. It's even worse for short trips in the winter. If you're just driving three or four miles on a cold winter day, you can expect a possible drop of 24%. You might think it would be better for hybrid vehicles, but it's actually worse. Gas mileage for hybrids can decrease by 30 to 40% in cold temperatures. And EVs do even worse than hybrids during the winter.

You gas mileage can drop during the dog days of summer, as well, but that's not directly caused by hot temperatures. It's the use of air conditioning that kills fuel efficiency. The EPA says that fuel economy for conventional vehicles can go down more than 25% with heavy A/C use. Just like with cold weather, short trips are harder on gas mileage than longer ones when using air conditioning. Surprisingly, according to a study cited by the EPA, the more efficient the drivetrain, the greater the impact of air conditioner use on gas mileage.

Why are cold temperatures so hard on fuel efficiency? There are lots of reasons for this. Your car's engine relies on oil and other fluids to stay lubricated. In extreme cold, these fluids are thicker, which means there is more friction between engine parts, causing them to work harder. Batteries don't work as well in cold temperatures, and that puts a greater load on your alternator. And think about the extra things you have to turn on in cold weather — the heater, window defrosters, heated seats, all things which consume power and put a greater strain on your vehicle. Cold temperatures also reduce the air pressure in your tires, giving you less traction and thus forcing the wheels to turn more. That's why you should always check your tires after a cold snap. Of course, we also use more gas when we idle the car to warm it up.

Just interacting with the environment itself reduces your car's gas mileage. Even the air makes it hard on your vehicle, since cold air is denser than warm air. Your car has to use more power to push through it. Snow, slushy, or icy roads force the car to use more power, as well.

Short trips are harder, because your car doesn't get a chance to stay at its normal operating temperature. On longer trips, the engine gets up to that temperature and maintains it for the duration.

Did you know that hot temperatures are better on gas mileage, according to the EPA? Warm air is less dense, causing less wind resistance. It's easier for your car's engine to warm up quickly to its most-efficient running temperature. Even the gas sold during the summer is more efficient than the blend sold for winters.

However, for cars fitted with air conditioning, that isn't always the case. According to one study, "Effect of Air-Conditioning on Light Duty Gasoline Vehicles Fuel Economy", air conditioning is the "largest auxiliary engine load for light duty gasoline vehicles". Fortunately, the EPA's estimate of a 25% drop in gas mileage doesn't apply in all circumstances. If you do a lot of idling or short trips, you might expect that kind of impact. But the study above found that when several different driving cycles are averaged, the effect on gas mileage is a drop of 1.3% to 7.5%. Interestingly, light-colored cars may use less gas since they reflect heat and are easier to cool.

What if you turn off the A/C and roll your windows down? It depends on your speed and the vehicle you're driving. A 2013 study, "Effects of Air Conditioner Use on Real-World Fuel Economy", compared driving with A/C to rolling the windows down, using a 2009 Toyota Corolla and a 2009 Ford Explorer. Both had better gas mileage with the windows down at lower speeds, but the Corolla was just as inefficient with the windows down past 75 mph. So your mileage may vary.