It's summer, which means many drivers will be cranking up the AC or rolling down the windows to keep their cars from turning into mobile toaster ovens. Which method uses more gas? In a column first published last summer and reprinted below, the Green Lantern explained that it depends on how fast you're driving.
Like most people, I crank up my car's air conditioning on hot summer days. But my wife has recently been tsk-tsking me for this practice—she says the AC wastes too much $4-a-gallon gas, and that we should roll down the windows instead. But I've read that rolled-down windows also decrease fuel economy, since they increase drag. 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 resort to air conditioning when you hit the highway. Every car has a speed at which rolled-down windows cause so much drag as to decrease 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 posted speed limits.
Your wife is certainly correct that air conditioners sap power from the engine and increase gas consumption. Depending on your vehicle's design, an active AC can cut fuel economy by anywhere from 3 percent to 10 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, at least, the fuel-economy losses 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 (PDF) this issue back in 2004, using both a wind tunnel and test track in Mesa, Ariz. 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 a lot of 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.
The Lantern would be curious to see this same test run for smaller cars designed with fuel savings in mind, rather than V8 behemoths. Paradoxically, 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. (As the SAE researchers noted, 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 Lantern can't vouch for the accuracy of either estimate, as there are no scientific data to analyze, but 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, the Lantern realizes that 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. Having once made a punishing mid-June drive from Las Vegas to Los Angeles sans air conditioning, the Lantern knows that natural cooling isn't always as effective as man-made.
So stick with the rule of thumb mentioned in the first paragraph, 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.
Is there an environmental quandary that's been keeping you up at night? Send it to firstname.lastname@example.org, and check this space every Tuesday.