GM Guy has the best no-cost involved advice. I have a 94 K2500 vin F, auto tranny, 4.10 gears, 265/75/16 Toyo M55 tires, Holset HX35W turbo, 4" exhaust, stock tune, and cab height canopy. The best I've ever been able to get is 15.5 MPG. That was with summer diesel and all highway miles. Once on...