C3500
454C
4l80e
4.10 rear
Since I've owned it (105k), I've rebuilt the trans + **** kit, replaced all the following; new lifters, injectors, ignition sys, cooling system.
245/75r16
4" spindle
I've never seen more than 10mpg.
Drove up and down from Cali to Montana last summer with camper, same...