Your MPG will depend on your vehicle weight, air pressure in your tires, age of the engine (under 60,000 miles is still "break-in" mileage), your rear end settings, your transmission settings and your driving speed.
You could expect anywhere from 6 to 11 mpg.
My rig is a Cummins C8.3 mechanical engine (before electronics) turning an Allison MD3060 tranny, pushing my '98 Dutchstar DP on a Spartan Mountain Master chassis. Since purchasing the rig in '03 I have averaged 9.25 MPG.
Fulltimers! - Currently in Gainesville, FL - working as Webmaster & Special Projects Manager