I don't see much of a change when I use the AC, but I drive about 20-25 miles every time I get in the car. When I crank it up at first to cool the car, it does drop the milage, maybe by 0.4 or, if a light is long and I can't do autostop, 0.8. But I scale back the AC as I go until I'm running with the air on 70 degrees and lowest setting after Auto, and I make up the mpg difference. When I'm using the low settings and driving steadily on a flat freeway, I see no difference between having the AC on and having it off. I'm still getting my 48 mpg this tank.
--A