All of the results from the T.V. show MYTHBUSTERS are listed here:
Running a car with air conditioning on is more fuel efficient than running with the windows down. (From episode 22)
The fundamental flaw in the MythBusters' test was that the point where the drag becomes powerful enough to inhibit a car's performance with windows down was inside their 45 - 55mph margin at 50mph. Going less than 50mph it is more efficient to leave your windows down, but going greater than 50mph it is more efficient to use your A/C.