ORIGINAL: purnrg
I saw an episode of Mythbusters on the Ac/Windows down comparison (they used a ford expedition) and the conclusion was that driving with the windows down at all speeds saves fuel over using the AC by a significant margin, if I remember correctly they got 2 3/4 extra laps on the test track they used with the windows down.
You are partially correct. Consumer Reports' auto-test department reports that the air conditioner reduces your car's fuel efficiency by up to 10 percent. So to achieve maximum fuel efficiency, drivers should avoid using the air conditioner at speeds below 40 mph and travel with their windows down. Howver, Mythbusters found that as your speed increases to 45 mph, or highway speeds, wind drag becomes an issue. Driving with the windows down increases the drag on your vehicle, resulting in decreased fuel economy by up to 10 percent. Drive at speeds over 55 mph with windows down and you'll decrease fuel economy by up to 20 percent or greater.