Somewhere there was a Mythbusters episode about a/c vs open window. Unfortunately I cant remember what the outcome was
Edit:Found it!
http://kwc.org/mythbusters/2004/11/mythbusters_boom_lift_catapult.html
<h3>Windows down vs. air conditioning</h3>
"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.
Mythbusted
Edited by: bazves