Can I throw another wrinkle in here? During the Summer, when I get my best mileage, I use my AC, alot. (I paid for it, it doesn't leak, and.... it feels good!) Shouldn't my mileage suffer from use of the AC?
Yes, AC does impact gas mileage as it robs power, but that is probably made up by the better gas that is sold in the summer months.
Thats my guess anyway. I also think that at highway speeds, the AC running doesn't impact mileage all that much. I have seen some arguments that its actually more efficient to run the AC than to roll your windows down - lower drag.