Conservation 101: the Compete to Conserve Blog

photo credit: emdot

I’ll admit that there is nothing like coasting down the highway on a hot, hot day with the windows down and the music blaring. I don’t even like air conditioning. But since the kid came along, I’ve found A.C. to be a necessity.

The one issue: Kissing all of that extra fuel goodbye.

When Bankrate.com published a story recommending readers to go easy on A.C. and to enjoy the summer air, the onslaught on their email inbox was astounding. Although the site stands by the statistic that “Air conditioning can drag down your car’s fuel economy by 10 percent to 20 percent,” the insistence from readers that using AC saves gas in the end drove them to dig further.

The debate circles around drag—the force exerted on an aerodynamic body that reduces forward motion. Rolling down the windows of your car intensifies this force, especially as speed increases.

The Society of Automotive Engineers (SAE) did a study in 2004 to determine when the drag of open windows sucks more fuel than using A.C. The study concentrated on SUVs and full-size sedans, so the effect on newer, smaller, fuel-efficient car models is yet to be determined, but what they found does provide some answers.

The report concluded that rolling down the windows has “a significant real impact on fuel consumption.” But the extent of the impact depends on temperature, vehicle speed, vehicle size, and wind velocity and direction. Overall, the SAE found that most cars were more efficient with windows down until a cruising speed of about 65 mph. At highway speeds the difference between using AC and rolling down windows became negligible.

Hence, the US Department of Energy advises this rule of thumb: When driving around town, roll windows down. When flying down the highway, opting for AC is okay.

This rule of thumb doesn’t necessarily apply to SUVs, however, because their boxy shape creates a lot of drag all on its own. So rolling down the windows may always be the better option for SUV fuel-efficiency, according to the SAE.

Before having to decide what works for you, there are ways to keep your car cool without either option. Here are some tips (care of Bankrate.com and Stanford University’s School of Earth Sciences):

-       Park inside a garage, under a tree, or in another shaded area.

-       Use windshield shades.

-       Open windows and vents to blow hot air out before turning AC on.

-       Use fan to cool vehicle—fan speed doesn’t affect fuel economy.

-       Recycle air inside vehicle, rather than pulling in hot outside air.

-       When using AC, opt for short burst, then re-circulate cool air inside car.

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