Would you get better gas mileage on a long trip with the ac on or windows down?
Ac off, windows down
windows down - fact.
What about the wind drag? In NASCAR Racing a small piece of duct tape over the radiator opening will make the car much faster for the same amount of fuel.
Windows down at slow speeds (city driving). a/c at freeway speeds.
For an in-depth answer to your question... http://auto.howstuffworks.com/fuel-efficiency/hybrid-technology/driving-with-windows-down.htm
Ac on , myth busters did this