It’s a hot day, you’ve got a long drive, and your mind is wandering. Ever wondered whether it’s more fuel efficient to drive with windows down or up? Air conditioning on or off? So did I!
For a while, I was under the impression that turning on the car’s AC was bad because it burns extra gasoline. This is true: using AC increases fuel consumption by about 10%. But it’s also true that opening your windows, especially at high speeds, increases the wind resistance on your car, which increases your car’s fuel consumption. At certain speeds, opening your windows also increases your fuel consumption by 10%. So what is the tradeoff here?
As it turns out, the break-point is somewhere between 40 and 45 mph. At 40 mph, it makes more energy sense to roll down the windows, and at 45 mph, it is more sensible to turn on the AC and keep the windows up. This is not to say you should alternate between using the AC and rolling down the windows every time you switch between 40 and 45 mph (that could get pretty tricky and time-consuming), but as a general rule, keep the AC for highway driving, and leave your windows down for everything else.