Does it really save gas to roll down your windows instead of flipping on the AC?
It’s summer, and it’s scorching hot. You gotta stay cool in your ride. This means many drivers will be cranking up the AC or rolling down the windows to keep their cars from turning into mobile baked potatoes. So, which method uses up more gas? Well after studying both methods, it depends on how fast you’re driving.
Like most people, we crank up the car’s air conditioning on hot summer days. But some argue that AC wastes too much gas and that we should roll down the windows instead. I’ve personally read that rolled-down windows also decrease fuel economy, since they seriously increase drag. So…what’s the most efficient way to cool ourselves while driving?
The rule of thumb is to keep the windows down while on city streets, then use the AC when you hit the highway. Every car has a speed at which having the windows down will cause so much drag that it decreases fuel economy more than a switched-on AC. As you might expect, however, that milestone speed varies widely from car to car—and in some cases, it may be well north of those speed limits.
It is correct that air conditioners drain power from the engine and increase gas consumption. But depending on your vehicle’s design, an active AC will cut fuel economy by anywhere from three percent to ten percent in standard summertime temperatures. During a brutal heat wave though, the power drain can be near 20 percent—the hotter it is outside, the harder the AC needs to work at maintaining your cabin climate. It’s worth noting here that automotive air conditioners no longer use ozone-depleting chlorofluorocarbons; now they use much safer tetrafluoroethane.
At low speeds however, the fuel-economy loss associated with rolling down your windows are minuscule. But as your foot gets heavier on the accelerator, the situation rapidly begins to worsen. That’s because drag increases with the square of speed. So when you hit the highway, all that wind whipping through your open windows begins to take a major toll. Even with the windows sealed tight, the majority of your car’s power goes toward fighting wind resistance when you’re cruising at 55 miles per hour. With the windows down, the engine really starts to strain.
But at what exact point do the numbers tilt in favor of air conditioning? The Society of Automotive Engineers studied this issue back in 2004. The organization’s researchers looked at two vehicles: an SUV and a full-size sedan, both of which featured powerful eight-cylinder engines. The tests were conducted at an average ambient temperature of approximately 86 degrees Fahrenheit.
The engineers found that rolling down the windows on the SUV had only a small negative effect, in part because the vehicle’s big, boxy shape was already creating so much drag. So, from a fuel-economy standpoint, a driver of an SUV will always do better to shut off the air-conditioner. The sedan, on the other hand, has a sleeker shape and a lower drag coefficient. As a result, its fuel economy was noticeably affected when the windows were rolled down at highway speeds; at around 68 miles per hour (the test’s maximum), there was barely any difference between air conditioning and nature’s cooling. If you were driving the sedan any faster than that, the increased drag would presumably make AC the more efficient option.
It would be interesting to see this same test run for smaller cars that are designed with fuel savings in mind, rather than the V8 behemoths they used. Because many fuel-efficient vehicles have low drag coefficients, they may actually experience larger relative increases in drag when the windows are rolled down at high speeds. The sedan’s drag increased by 20 percent with the windows rolled down, versus just 8 percent for the SUV.
Some engineers have claimed that 45 miles per hour is the break-even threshold for average-size cars; others put the figure closer to 75 miles per hour. The truth may lie somewhere in the middle. Of course, that “truth” may vary according to such factors as ambient temperature and wind velocity.
Even if rolled-down, windows eke out a win at all but the highest speeds, yet many drivers will balk at the idea of zipping along the highway while exposed to the elements. There are safety issues to consider, as well as noise and general comfort.
So stick with the rule of thumb mentioned earlier and you should save a few gallons of gas over the course of the summer—though not nearly as much as if you decided to cut down on your driving a bit. You needn’t feel too guilty about bathing in the air-conditioned splendor of a mass-transit vehicle. There just isn’t a big enough difference to feel guilty about cranking that AC!! I say blast it if you have it. That shitty 1989 Toyota Celica sitting next to you at the red light WISHES he had AC to even have this dilemma!!