When I was learning how to drive, I was taught that air-conditioning was this enormous drag on gas mileage. I might as well be towing around an anvil behind the car. If I wanted better gas mileage I had to use the wind from the window.
Now it turns out that's not quite right. An air conditioner can up cut 2 mpg, according to American Petroleum Institute. But that's not nearly as much as the drag from your window will cut it. You're better off putting on the air at highway speeds, says the American Aftermarket Suppliers Association. According the the AAA, keeping windows closed at highway speeds can cut your fuel usage by 10%. But in stop and go traffic, you should keep your windows down and the air off.
Not everyone completely agrees. Edmunds did a study and found that on at least one pick-up, the Toyota Tundra, using the windows instead of the air improved gas mileage by nearly 10%. They chalked up the difference to the peculiarities of pick-up aerodynamics.
So, the old debate of windows or air can still be fought in car seats across the country. It sounds like the most economical solution is what people do just to enjoy the breeze but keep their hair from getting messed up: windows down at slow speeds, air conditioning on the highway.
When air-conditioning actually gets you better gas mileage