How Badly Am I Killing My Gas Mileage Driving with My Windows down vs. Driving with the A/C On?

Or: How do I cool myself in the most fuel-efficient way possible?

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If the modern automobile had a status on Facebook, it would undoubtedly be “It’s complicated.” With their computer-controlled fuel injection systems, continuously variable transmissions and three-phase four-pole AC induction motors, the days when every Dick, John and Harry could wrench on their ride seem long gone.

But you don’t need to be a gearhead to master the basics. In fact, despite cars’ complicated nature, not much has changed when it comes to the simple stuff — and when it comes to Car Knowledge 101, there are no questions too dumb.

I drive around all the time with my windows down because I prefer the breeze in my face to the stuffiness inside my car. So what I want to know is, how bad is it for my gas mileage? Am I going to save a significant amount of money rolling them up and turning on the A/C instead?
Who doesn’t enjoy a little wind through the hair? I’m right there with you, bud — personally, there’s something about hanging an elbow out the car that just does it for my vehicular claustrophobia. And unlike your A/C, which requires energy from your engine to operate, and burns gas to create that energy, it’s Mother Nature’s way of keeping you cool. But bad news, amigo: Keeping your windows rolled down can significantly affect your gas mileage, depending on a couple of factors.

First, it’s important to understand that the reason rolling your windows down affects your gas mileage at all is because of drag. Car manufacturers spend millions, if not billions, designing their rides to move through the air as efficiently as possible. Poorly designed aerodynamics mean that, instead of cutting like a knife through the air, a car will operate more like a parachute, catching the wind and slowing the vehicle down. At higher speeds, cars with a higher drag coefficient require more gas to offset the loss of speed due to wind resistance. So unfortunately, if you roll down your windows, you’re messing with the aerodynamics the manufacturer spent all that money designing, because wind that would normally flow over the car is causing drag swirling around your interior instead.

That little bit of knowledge is important for the first factor: The speed at which you’re driving. At low speeds, where aerodynamics don’t really come into play, rolling down your windows has little to no effect on your gas mileage because of the minuscule amount of wind resistance. But as you pick up the pace, your car’s rolled-down windows will suck more and more wind into the interior, creating more and more drag. According to a study by the Society of Automotive Engineers (SAE), when driving above 50 miles per hour, that drag can reduce your fuel efficiency by 20 percent.

The second factor is the type of vehicle you’re driving — specifically a car or an SUV. If you’re hitting the highway in a Porsche (a highly aerodynamic sports car) your fuel mileage might get hit with that 20 percent efficiency reduction or worse with the windows rolled down. But if you’re in a Jeep Wrangler, which is essentially a wind-resisting box on wheels, your mileage might only be reduced by 10 percent, because your Jeep isn’t aerodynamic in the first place.

Now, air conditioning, on the other hand, causes your car to burn fuel at the same rate whether you’re going 10 miles per hour, or 100. That’s because at 10 miles per hour your A/C unit puts the same load on the engine as it does if you’re going 100. But if it’s a hot day, look out — that load you’re putting on your engine is going to be higher if you’ve got your A/C turned up to 11, versus if it’s at, say, 1.

So which is worse for your fuel mileage, windows down or A/C?

Well, according to that SAE study, your A/C is worse at low speeds, when the wind resistance is low and your car isn’t acting like an air brake. And it’s also definitely worse than rolling down your windows in an SUV, which is already fuel inefficient because of its shape. But at high speeds, say in the 50 to 75 miles per hour range depending on the car you’re driving, there comes a point where air conditioning is better for your mileage than rolling down the windows.

The quick-and-dirty bottom line: If you’re on the streets, roll your windows down. If you’re on the highway, roll ‘em up and crank the air — just not too high.