same here 3.73 with a zf6 and 37s i get about 17 if i go long distance but for the most part 15.5 sometime 16 just driving to work and back. when i tow i get about 15 . but im only towing a jeep. honestly if i were to put gears id go with 4.10s these trucks have to power to pull 38s4.30s will put you right where the stock powerband and power would be when running 37s or 38s. 4.56s will up the rpm a bit on the highway but give you a little more at take off. I run 37s with my 3.73s and love the highway running at 17mpg and when towing I just lock out the od and at 65mph it tachs at 2100rpm and purrs right along. When using od at 65 I tach at 1700 and if not in the hills ill get 17mpg in hills I lock out od and let it sing.