Ive got an 06 5.9 mega cab 2500. 100hp sticks industrial injection fuel return line black, and a bad homade air intake from the previous owner, also sperchip tuner on fuel econ. It gets horrible mileage. 14 city 8.5-9 pulling my enclosed trailer. I pulled the same trailer with my 04 cummins and got 13-14 pulling the trailer. The old truck was pushing 8 inches of lift and 37s, not sure why this 06 sucks fuel. Im not complaining about city mileage mostly just pulling mileage. Anyone have any ideas?

 

Might your injectors not be playing nice with the tuner?

What are your EGT's while toing?