My personal best was 34mpg on my way down to minneapolis for a rallyX, with a detour well out of the way to sell some wheels. If I had to guess, it was approximately 120 miles of 70mph, and another 140 of 55mph / slowing for towns. Can't remember how much fuel was left in the tank, but it wasn't even close to being on fumes when I got done. Probably a tailwind though.
Typical DD mixed driving is between 22-25, and typical highway hovers around 29-30. 5-speed.