AC vs. Windows Down
Adam and Jamie tackle not so much a myth as what they call an "urban puzzle". The debate arises because both methods of cooling influence a car's fuel efficiency—air conditioning requires a lot of power to run, but at the same time, open windows create drag.
Revisited in MythBusters Revisited, Season 3, episode 38
Myth statement Status Notes
Running a car with air conditioning on is more fuel efficient than running with the windows down. Partly Busted Tests were performed under varying conditions (55 miles per hour (89 km/h) versus 45 miles per hour (72 km/h)). Also, the 55 mph test was using a computer to estimate fuel efficiency based on air intake, not actual fuel consumption, and showed A/C was more efficient. The 45 mph test consisted of running the tank until it was empty, and showed open windows were more efficient. This experiment—or one like it—is sometimes cited by the Magliozzi Brothers on Car Talk when presented with this question.