My car (2007 Mazdaspeed 6) has a 25MPG EPA rating for highway. It can get close to that going 75-80MPH. Going 65MPH, it can get higher (up to 30MPG). I calculated this a few years ago on a long road trip (WI to CO to CA to CO to MI to WI) by keeping track of fill-ups and mileage.
Glad I am not the only one that wondered how I would pull 450+miles out of a 15 gallon tank that should be getting 25mpg or about 375miles a tank... 75+ mph trips