I bought a 99 Cummins with ATS automatic. I installed a TST Power Max, K+N filter/intake and MBRP exhaust and I get a constant 20-21 mpg with leveling kit and 315's. I went to 315's because mine had factory 4.11's and was pulling to high RPM's with small tires. You travel very much in CTD above 2K rpm's the fuel mileage REALLY drops. I also don't use the box for everyday driving I run it in fuel economy mode. I just drove from WA to CO and got 713 (gps measured) miles before the low fuel light came on.
Shamb you can't go wrong with a CTD.
My father and brother n law both have 7.3's and they flip my crap all the time but to each their own.
Good luck in the tow rig hunt.
