Whether to Turn on the A/C or Roll Down the Windows, According to Science
Rolling down the windows seems like the most eco-friendly and economical way to cool off when you're driving. You don't have to turn on the air conditioning, because you get a nice breeze from nature itself. Right?
Well, not exactly. The air that rushes into your open windows is doing so because your car is moving so quickly. And you're using fuel to keep your car moving. So it makes another issue: Opening the windows makes your car less aerodynamic, and thus less fuel efficient.
The best option depends on your speed
There have been experiments over the years trying to sort out which cooling option saves the most gas, with varying results. A 2004 study from General Motors and SAE found that turning the A/C on uses more gas than rolling the windows down, with a more significant difference for the SUV they tested than for the sedan.
But the same study also noted that there were a lot of variables that affect fuel usage between the two conditions. The speed you're driving is a biggie, but other factors include the temperature, wind velocity, and various aspects of how the car was designed, including the engine's efficiency.
Mythbusters tackled the question as well, finding different results in a test at 45 miles per hour and at 55 miles per hour. They revisited the question in a 2005 episode, finding that 50 miles per hour seems to be the crossover point: above that speed, rolling the windows up saves more gas than the air conditioning consumes.
Another test, this one from Oak Ridge National Laboratories, found that, in a 2009 Toyota Corolla, A/C beats rolling the windows down once you're driving at 75 miles per hour or faster. In a 2009 Ford Explorer, both options were equivalent at highway speeds.
Roll the windows down for local roads, and roll them up on the highway
While there's variation in the results, enough of a pattern emerges to give us a rule of thumb. The U.S. Department of Energy sums it up in this recommendation:
Lower the windows in your vehicle when driving at lower speeds, and use the air conditioning system when driving at highway speeds.
They also point out that you should roll down the windows when you first get into a hot car, to allow the hottest air to escape. Then you can let the air conditioner get to work cooling the warm air that remains.
Put all together, the smartest way to cool yourself down on a summer's day is to drive with the windows down at first. If you stay on lower-speed roads, that may be all you need to do. But when you're approaching a highway on-ramp, that's a good time to roll up the windows and finally turn the A/C on. Closing the windows also cuts down on highway noise, saving your ears and making it easier to hear your road trip music or podcasts.