I get 28-29mpg at 75 on a perfect flat highway, with no wind.
23-25mpg at 80mph on same conditions

I have noticed that wind speed has a dramatic effect on fuel economy.

This is all based of my dash command

**edit: this is with my spoiler down. I took the fuse out a while ago, 80lbs of downforce and 80lbs of drag is not necesary on th highway, its like carrying another person