I've never found the A/C to make much of a difference on mine. For smaller cars with smaller engines, you can feel the drag it causes. In town, it might cost you 1-2 mpg, but on the highway, i bet it's not much. Rolling down the windows chews up more than the A/C does.

Didn't the Mythbusters do a bunch of testing on how the A/C affects mpg already?

 

In town roll them down.
On the highway use the AC.

 

when I drive my straight drive truck, i will also cut the engine completely off going down hills. My cousin does the same and he sees 3mpg difference from it. I dont drive mine enough to check. Just remember that steering and braking is alot harder. And dont turn the key to the lock position.

On large hills I will leave it in gear but kill ignition. That way I still have brakes and steering is easier.