2000 k3500 GMC SCLB SRW with 350 and 4l80 trans, 4.10 gears, 255-85 r16 Cooper ST's. EGR and cat delete have not changed fuel economy, also has not been significantly retuned - near future modification.
Best fuel economy not towing- 14.7mpg.
Once coming out of the mountains of Colorado with the cruise ~85mph+, AC on, motorcycle + test equipment, two guys, est 8,000lbs. Several times unloaded (~6100lbs) back and forth to work, keeping speed ~67mph plus stop signs on the back roads, never over 2200rpm, always on summer fuel.
13.5-14 when running 72-75. 13-13.5 when running 85-90 with cruise on.
12.5ish commuting to work with winter fuel.
Best economy towing -12.5 mpg.
Towing a pickup with combined weight of ~12k. Significant excursions with foot to the floor, never broke 60 as all two-lane highway driving.
Worst economy towing - 11.0mpg. Towing Unimog with the aerodynamics of a house at 14klbs+ combined and my right foot on the loud pedal in the radiator. 65-70mph.