Yeah, I'll bet the speed is going to get me more than anything else. Just for fun, what would you guess the difference in mpg would be between:

A. 75 mph with the a/c on for 3 hours of flat interstate, and
B. 65 mph with no a/c on for 3 hours of flat interstate?

I have no idea. Maybe I'll drive to Louisville using Option A and drive back using Option B and find out!