[fredjacksonsan]

The AC vs windows debate is an old one. If you're constantly moving at high speed, then the AC on with windows up gives the better mileage, as wind resistance grows with speed while the AC resistance stays pretty constant.

However in stop and go or at lower speeds, the situation is reversed, as the constant drain of the AC is greater than the variable wind resistance. Each vehicle has a different speed for AC vs. wind resistance, but figure that most vehicles traveling less than about 45-50, (where wind resistance builds rapidly) get better mileage with the windows down and AC off.