Air conditioner vs. rolling down a car's windows: What's more efficient?

Monday, May 26, 2014, 7:32 PM - Research by the website vox.com argues that it's always more fuel efficient to roll down your car's windows in lieu of air conditioner, regardless of the speed you're driving at.

When it comes to a car's fuel efficiency, the air conditioner vs. open windows debate is nothing new -- and it gears up (pardon the pun) every year as warmer temperatures settle in.

Vox.com is attempting to put an end to the age-old controversy.

The website has posted an extensive article arguing that it's always better to drive with the windows rolled down, regardless of the speed you're travelling at.

"Conventional wisdom holds that open windows are better at slow speeds, and air conditioning is better when you're flying down the highway," the article reads.

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"But the surprising thing is that actual tests of the idea have arrived at a different result. Good real-world data is frustratingly limited, but the few tests that have been conducted say if you can deal with the sound, it's slightly more efficient at any speed to roll down the windows, even on the highway."

The website cites an article by slate.com which argues that a standard air conditioner can cut fuel efficiency by as much as 10 percent -- and that can jump to 20 percent during a heat wave because, according to slate, "the hotter it is outside, the harder the AC needs to work at maintaining ... cabin climate." 

The long-held belief that air conditioner is more fuel efficient at highway speeds is due to drag.

It has been theorized that open windows at a fast speed make a car less aerodynamic, and that forces the engine to work harder. In doing so, it uses more fuel. Check out this episode of Mythbusters that came to a similar conclusion:

Apparently not content with their findings, the Mythbusters team re-visited the theory, discovering that rolled down windows may, in fact, be best.

"But they weirdly changed speeds halfway through the tests, and messed with other variables, making analysis pretty difficult," Vox says.

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Vox cites several sources to back up its claim, including a 2004 study by the Society of Automotive Engineers which argues that, while the amount of gas consumed by an air conditioner vs. open windows may be negligible in some larger car models, fuel efficiency is always better with the windows down.

But if you're looking for the best way to maximize your car's fuel efficiency, "windows up and AC off is always more efficient, since there’s no drag and no power drain from the air conditioner," says Weather Network digital meteorologist Scott Sutherland.