Best MPG grossing 117,000 pounds

I still run everyday with a 2007 KW T800B C15 Acert, Engine Brake & Retarder, 18 sp, Eaton 2 sp 46K rears (3:90) the Engine was done @ 715,000 the Truck now has 1,051,000 on it being the only mechanical work done is the Engine, Clutch (514,000) and regular maintenance, I am the only one that has ever driven this Truck, average 120-130k gross.....5.4-6.3 mpg.