Power to weight ratio has a lot to do with it too. If you have relatively high power vehicle with a relatively low weight (12-15 lbs per hp), as long as it is not geared too steep or accelerating too hard to get up to speed, decent fuel economy is essentially a given, and while youre right wind resistance does increase geometrically, it doesn't overcompensate for a decent power/weight ratio in detracting from fuel economy. Add the higher speed/fuel efficency powerband, and the fuel efficency factor is not adversely effected by speed unless you are way up there in velocity, like 95+mph or more.
From Nashville TN to Baton Rouge, LA, going I-40, I-55, and I-12/10, doing 65 on the trip down I averaged 20.98 mpg. From Baton Rouge back to Nashville, I did 75-85 most of the way, depending on where I was (didn't do much over 80 in TN for fear of the lawman), and I got 26.35 mpg.
Power to weight ratio doesn't effect fuel economy at cruising speeds. Power to weight ratios are a determining factor in the fuel burned to reach that speed.