wind resistance. at 80 mph, I think it's half of your gas is going into fighting wind resistance (don't quote me on it though). if you were to graph the amount of wind resistance to speed, it would be something similar to an exponential curve, as you go faster, 1 mph makes that much more of a difference. however, the contour doesn't really get 30 mpg as a general rule (at least mine doesn't...used to get 18ish, but this summer it hopped up to 21ish, and is dropping back down...
) so don't worry too much.
