Just got back from an 1100 mile round trip with my first TT, and as others have said - it's almost entirely drag-related.
2470 Springdale (just under 30') @ 6000lbs, somehow averaged around 10-10.7MPG with a 2006 land rover LR3 (4.4l / 6 speed). BUT I had to go through the WV mountains / I-64 the entire way (VA to KY!)
The second a tractor trailer would pass me and get in front my MPG would leap to ~16+
I think we all see similar numbers due to drag (increases exponentially with speed) - after a certain point it's just all going to average out similar.
