Mythbusters actually tested this. Here's the thing though, according to the "quiz" on their website, windows up A/C on gets better fuel economy than windows down A/C off:
http://dsc.discovery.com/games-quizzes/driving-myths/ However, according to their test, it's the exact opposite by a significant margin:
http://dsc.discovery.com/videos/myth...-minimyth.html