Whether to Turn on the Air Conditioner or Roll Down the Windows, According to Science

Rolling down windows seems to be the most environmentally friendly and economical way to cool off while driving. You do not need to turn on the air conditioner because you get a pleasant breeze from nature itself. Right?

Well, not really. The air that rushes into your open windows does so because your car is moving so fast. And you use fuel to keep your car moving. So this creates another problem: opening windows makes your car less aerodynamic and therefore less fuel efficient.

The best option depends on your speed

Experiments have been conducted over the years trying to figure out which refrigeration option saves the most gas, but with varying results. A study by General Motors and SAE in 2004 found that turning on the air conditioner used more gas than rolling down the windows, with the difference between the SUVs tested being more significant than that of the sedan.

But the same study also notes that there are many variables that affect fuel consumption in these two conditions. The speed at which you drive matters a lot, but other factors include temperature, wind speed, and various aspects of vehicle design, including engine efficiency.

The MythBusters also tackled this issue, with different results in the 45 mph and 55 mph test. They revisited the question in a 2005 episode and found that 50 mph seems to be a crossover point: above that speed, raising the windows saves more gas than the air conditioner uses.

Another test , conducted by Oak Ridge National Laboratory, found that in a 2009 Toyota Corolla, the air conditioning outruns the windows down when you are driving at 75 mph or faster. In the 2009 Ford Explorer, both options were equivalent at highway speeds.

Roll down the windows on local roads and roll them up on the highway.

Even though the results vary, enough patterns emerge to give us a rule of thumb. The US Department of Energy summarizes this in this recommendation:

Roll down your car windows when driving at low speeds and use the air conditioning system when driving on the highway.

They also note that you should roll down the windows when you first get into a hot car to let the hottest air out. You can then let the air conditioner run, cooling the remaining warm air.

In general, the best way to cool off on a summer day is to drive with the windows down first. If you stay on slower roads, this may be all you need to do. But as you approach the highway exit, it’s time to roll up the windows and finally turn on the air conditioner. Closing the windows also reduces road noise, saving your ears and making it easier to listen to music or podcasts from your trip.