A/C or Open Windows? How to Stay Cool on a Summer Road Trip Without Wasting Gas

Every summer road trip brings the same small debate in the front seat.

One person wants the windows down for fresh air. The other wants the air conditioning on high.

Both choices affect comfort, and both affect how much fuel you burn on a long drive.

This guide uses research from FuelEconomy.gov to explain when each option makes sense and how to keep the cabin cool without wasting gas.

Hot weather is not bad for fuel economy by itself

It may sound surprising, but warm weather can actually help your gas mileage.

According to FuelEconomy.gov, the engine warms up to an efficient temperature faster in hot weather.

Summer grades of gasoline can also have slightly more energy, and warm air causes less aerodynamic drag than cold air.

The catch is what we do to stay comfortable.

Rolling down the windows or running the air conditioning can both reduce fuel economy.

How much the air conditioning costs you

FuelEconomy.gov calls the air conditioning the main contributor to reduced fuel economy in hot weather.

Under very hot conditions, using it can cut a conventional vehicle's fuel economy by more than 25%, particularly on short trips.

The effect on hybrids, plug-in hybrids and electric vehicles can be even larger on a percentage basis.

How much fuel the system uses depends on the outside temperature, the humidity and the intensity of the sun.

A humid afternoon in direct sun asks much more of the system than a dry, cloudy morning.

How much open windows cost you

Open windows are not free either.

They increase aerodynamic drag, which is the wind resistance the car has to push through.

FuelEconomy.gov notes that this effect is quite small at low speeds but increases at highway speeds.

That difference in speed is the key to settling the front-seat debate.

The simple rule: windows in town, A/C on the highway

The official advice is short.

  • At lower speeds, roll the windows down.
  • At highway speeds, use the air conditioning.

In town, the extra drag from open windows is small, so fresh air is the cheaper way to stay cool.

On the interstate, open windows create a lot of drag, so closing them and using the air conditioning makes more sense.

Closed windows also make highway driving quieter, which helps on a long day behind the wheel.

Cool the car faster when you first get in

The hardest moment is the first few minutes after the car has been parked in the sun.

FuelEconomy.gov offers a few habits that help.

  • Air it out first. Drive with the windows open for a short time before using the A/C. Letting hot air out puts less demand on the system and helps the car cool faster.
  • Do not idle with the A/C running. Turn it on after you begin to drive, or after airing out the cabin briefly. Most systems cool the car faster while driving.
  • Do not overcool. Avoid using the A/C more than needed or setting the temperature lower than needed.

Idling is wasteful in any season.

FuelEconomy.gov notes on its driving habits page that idling uses fuel while the car goes nowhere.

Keep the heat out while you are parked

The less heat the cabin absorbs, the less work the air conditioning has to do later.

  • Park in the shade whenever you can.
  • Use a sunshade on the windshield when you stop.
  • Choose a spot where the sun will be behind the car later in the day.

These are small habits, but on a trip with many stops they add up.

The car is more comfortable from the first minute, and the A/C reaches a cool temperature sooner.

If you drive a hybrid or an electric vehicle

Electrified vehicles feel the effect of the air conditioning more strongly, so planning helps.

FuelEconomy.gov suggests pre-cooling the cabin while the car is still plugged into the charger.

That uses power from the grid instead of the battery and can extend your driving range.

Using a warmer temperature setting for the A/C also uses less battery power.

On a long trip, that can mean one fewer charging stop.

Read the owner's manual for your system

Climate controls vary a lot from car to car.

FuelEconomy.gov points out that most manuals explain how the controls work and how to use and maintain the system.

Look for what the recirculation button does in your car and whether an automatic mode manages it for you.

Check whether the manual recommends a cabin air filter replacement interval.

A few minutes with the manual before the trip can make the system work better all summer.

Comfort is a safety issue too

Staying cool is not only about comfort or fuel.

A hot, stuffy cabin makes drivers tired and less alert, and fatigue is a real risk on long drives.

If you have to choose, choose a comfortable, alert driver over a few extra miles per gallon.

Bring plenty of water and take regular breaks in the shade.

Our guide on road trips in hot weather covers how to keep children, pets and passengers safe when the temperature climbs.

Other ways to save fuel on the same trip

The air conditioning is only one piece of your summer fuel bill.

Speed, extra weight, roof cargo and tire pressure all play a part.

Our guide on how to save fuel on a long road trip covers those factors with figures from FuelEconomy.gov.

Checking tire pressure before you leave is one of the easiest wins.

Our step-by-step guide on checking tire pressure shows how.

A quick summer cooling checklist

  • Park in the shade or use a sunshade at every stop.
  • Open the windows for a minute before turning on the A/C.
  • Start driving instead of idling to cool the car.
  • Use open windows at low speeds and the A/C on the highway.
  • Set a comfortable temperature, not the coldest one.
  • Pre-cool an electric car or plug-in hybrid while it charges.

The bottom line

There is no need to sweat through a road trip to save a little fuel.

Use open windows when you are driving slowly and the air conditioning when you are cruising on the highway.

Let the hot air out first, skip the idling and park in the shade.

Those habits keep everyone cool, alert and a little richer by the end of the trip.

Sources and Further Reading