Does Hot Weather Hurt a Car's Gas Mileage?

Dear Driving for Dollars,

It always seems like my gas mileage is much worse in the summer than it is in other seasons. How much does hot weather really hurt gas mileage and is there anything I can do about it?

- Phillip

Dear Phillip,

Have I got a surprise for you. Hot weather often improves a car's gas mileage.

Bo Saulsbury at the Center for Transportation Analysis explains that warm air causes less aerodynamic drag than cold air. It also helps your engine warm up to an efficient temperature faster, and summer grades of gasoline actually have more energy than winter grades.

So, what's going on with your car? It's a pretty safe bet that it has to do with your air conditioning. Running your air conditioner can decrease a traditional gasoline-powered car's mpg by more than 25 percent and even more for hybrids and plug-in hybrids, Saulsbury says.

Even though driving with your windows down creates increased aerodynamic drag, it will cause less impact on your fuel economy at low speeds. Saulsbury recommends keeping the windows down at low speeds and using the air conditioner when driving at highway speeds.

It also helps to park in the shade or use a sunshade to keep the car interior cooler and to roll the windows down for a short time before turning on the air conditioner to let the hot air out of the cabin first.

Ready for a getaway? Check out these seven best rental cars for summer vacation.

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