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It depends on how fast you are driving. Mythbusters did some tests, I beleive if you are under 65 mph then roll the windows down but over that speed turn the AC on... I will look for the link to that episode
Windows down vs. air conditioning"Urban puzzle": it is more efficient, on a hot day, to run with the A/C on and windows up than to run with windows down (b/c of increasing car's drag).Computer-based mpg measurements:11.7/11.8 with A/C on and windows up 11.7/11.8 with A/C off and windows up 11.3 with A/C off and windows down So, according to the computer, it's better to use A/C with windows up.This was too quick and easy for TV, so they decided to stage a seven hour marathon, race-til-you're-empty duel, with Jamie driving an SUV with A/C on and Adam driving an SUV with windows down. Though, once the safety inspector intervened, it was no longer a seven-hour marathon, it was a bit slower (45mph instead of 55mph), and a lot shorter (only 5 gallons each).Jamie's A/C car ran out of gas first -- Adam's windows down SUV ran for another 30 laps -- completely contradicting the computer mpg estimate. Computer estimate based on air flow into the engine, so it would appear that it is unable to properly model the difference between A/C and windows down.