I have been pondering the sweet spot for a while too.
I know my mileage plummets over 80mph, and is ok around 60, but set the cruise at 75 I will be getting 19, (with cap and 4wd) I think that the engine has a real easy time turning 3000rpm, and that wind resistance still is not so bad that it is eating up all of the power. I believe wind resistance goes up logarithmically (by the square of the speed) so at 20 mph wind resistance is 400, 40 mph it is 1600 and at70 it is 4900 and at 80 it is 6400 (I havent a clue about the unit) so a small increase in speed could be a massive increase in drag.....