The only thing that might make you more miserable than sweltering in high temperatures is paying high prices at the gas pump, and right now, millions of drivers are dealing with both.

The national average for a gallon of gas remains above $4, according to AAA, while large swaths of the country will be scorching under triple-digit temperatures for the Labor Day holiday.

That kind of heat isn't just uncomfortable — it's also draining your tank. Fortunately, there are some simple ways you can fight the heat and save some money on your next fill-up.

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(MORE: It's a summer heat wave, but your body thinks it's fall)

Your AC is the main culprit

Using the air conditioning in an average vehicle can reduce its fuel economy by more than 25%, especially on short trips, according to the Department of Energy.

• Roll down windows at low speeds: Stick to open windows when driving on local roads. Save the AC for highway driving, where open windows create aerodynamic drag that wastes more fuel than the air conditioner itself.
• Air out the cabin first: Roll down your windows for a minute before turning on the AC so your vehicle doesn't have to work as hard to push out trapped heat.
• Don't idle to cool down: Avoid sitting in your driveway waiting for the cabin to chill. Idling can burn up to half a gallon of gas per hour, and most cooling systems work better once the car is actually moving.

Tires matter more than you think

Hot temperatures will swing your tire pressure up and down. For every pound per square inch (PSI) your tires are underinflated, your fuel economy drops by roughly 0.2%. The correct pressure, on the other hand, can improve your gas mileage by up to 3%.

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Don't make this common mistake; the right pressure for your tires can be found on the sticker on the driver's side door jamb, inside the glove box or in the owner's manual. The number stamped on a tire's sidewall is the maximum tire pressure it can hold, not the recommended level for everyday driving.

Here's your heat-busting, gas-saving checklist

Higher temperatures cause fuel to evaporate faster, both in your tank and at the pump. So keep this checklist in mind to stretch every gallon:

At the pump and driveway:

• Fill up early in the morning or late at night when fuel in underground tanks is coolest
• Tighten your gas cap to prevent fuel vapors from leaking
• Park in the shade or use a reflective sunshade to keep interior temperatures down
• Check your tire pressure once a month (when tires are cold)

On the road:

• Drive with windows down at low speeds; switch to AC on the highway
• Avoid unnecessary idling
• Ease off the gas pedal — aggressive acceleration and hard braking can lower gas mileage by 15% to 30% at highway speeds
• Combine errands into a single trip

None of these fixes cost you a dime. And after the most expensive August ever at the pump, every gallon you don't burn is money that stays in your pocket.