Here is the exact information:
http://mythbustersresults.com/episode22
Running a car with air conditioning on is more fuel efficient than running with the windows down.
partly confirmed*
Tests were performed under varying conditions (55
mph versus 45
mph). The 55
mph test used 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.
*Because the original tests were inconclusive, this "urban puzzle" was revisited in episode 38:
It is more fuel efficient to use air conditioning when the car is traveling approximately 50mph or more.
Otherwise, windows are more fuel efficient.
My Rule of thumb - under 45mph roll the windows down - over 45mpg use the AC.