For long trips at highway speeds leaving the AC on won't really hurt much at all. But for daily driving with traffic lights and stop signs it will make a big difference in mpg. I took the belt off because something was squealing real bad. But I'm sure it picked up a couple of HP as well because the AC belt is tensioned and spins a weighted pulley on the AC compressor. The more I thing about it, it has to be at least 3 hp I picked up!
