The newer a/c compressors don't drag on the engine as they did back in the 60's and 70's, they take away very little hp. If the car is relatively drag efficient, then a/c and windows up would be more fuel efficient that no a/c and windows down. Now as for old cars and cars using the compressors that take a toll on the engine or less aerodynamic vehicles, then the efficency would be less likely. This does not apply to street rods and kustoms though. If you truly want to be cool in one of these cars, you gotta open the windows and hang your elbow out while wearing your shades and a tee shirt.

If you can't be Milner, at least you can pretend.