Bornyo quoted:

Quote:

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.
<snip>
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.

Mythbusted



OK, so this indicates that A/C takes a slice of efficiency. This is also my hands-on experience, even though it's far from extensive.
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