My 00 4x4 250, 7.3, auto, gets about 9mpg towing a 27', 3 ton TT. Not excessively large or heavy. It has about 185,000 on all original running gear. Other than the Zoo-Dad, a K&N stock replacement (going bye-bye soon) and FPR shim (to about 60 psi) the truck is bone stock. Does this mileage seem too low to anyone else? I tow about 35 miles in slight hills, the rest mostly flat interstate (I85 and 95), running between 60 and 65 (depends on traffic). I keep it in overdrive on the interstate, occassionaly cutting O/D off in the hilly parts.
She is mechanically sound and doesn't excessively smoke, even floored with the load attached.