Reply #12: That's not quite true

[Ravenseye]

The American TV show Mythbusters tested this using identical cars with the same amount of fuel. It found that at speeds up to 80 km/h it was more efficient to leave the windows open, and at higher speeds it was better to use air conditioning.

http://www.newscientist.com/backpage.ns?id=mg18925432.200

So if you're on the highway going 55 or faster for a long stretch of time, it's better to use the A/C, if you're in town and going slower it's better to roll down your windows.

Though it really depends on your car more than anything. Each car is different. More aerodynamic ones, are much better with the windows down, less aerodynamic ones, starts getting better with the A/C on.

It's not cut and dry.