It's an age old summertime driving debate: Is it more fuel efficient to drive with the windows rolled down, or with the air conditioner on?

Neither is good for your car's mpg numbers.

Opening the windows increases drag, and slows the car down — so it needs more fuel to run.

Running the A/C puts an extra load on the engine — so it needs more fuel to run.

In 2004, General Motors and SAE, which creates standards for various engineering industries, set out to find a definitive answer. The result is the report, "Affect [sic] of Windows Down on Vehicle Fuel Economy as compared to AC load."

The researchers used a wind tunnel and laps on a track to test a sedan and an SUV at speeds of 31 mph, 50 mph, and 68 mph, in 86 degree F weather.

As the two charts below show, running the A/C (the blue line) requires more gallons of gas per mile than driving with the windows open (green line). And if you really want to save money at the pump, roll the windows up, turn off the A/C, and sweat it out (red line).

 Here's how the numbers work out in the sedan, which a 4.6-liter V8 engine.

It's even clearer in the SUV, with an 8.1-liter V8 engine:

Not convinced? Discovery's "Mythbusters" tested this one out too, and found that windows open is the way to go.

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