WHAT kind of truck do you own ??

my SUBURBAN with a carberated 454 got 13 MPG CITY way back in the early 90's and it was a tire burner

My god, my NEIGHBORS 64 vette with a 528 ci street engine got 10 MPG average and it was pushing 970HP !!

A LTL9000(500HP turbo deisel) loaded to 80k GVW got better than your ride.

My suggestion is a used 4CYL car for the times you don't need to haul a camper or ride through rivers ;)