I have a 2006 T600 w/ 500 ISX, 13spd, 24.5's, 3.70 rears. Running 75mpg under 78,000lbs I got 4.85 the other day. I backed it down past Texas and got 6.2mpg. How you drive it depends on what mpg you get out of it.

I also have a flat top 84" bunk (no roof fairing) pulling a tri-axle moving van.