So many factors will change your fuel mileage. Here is what I have found out. SPEED is the determining factor in what kind of mileage you get. Slow it down to sip, speed up to drink. My truck came factory with the 18 inch wheels and would get 19-21 on the highway ( all things being equal) at 80 mph. I swapped to factory 20 inch wheels, that same 80 is now 16-17. Slow down to 70 with the 20s and it got 21-23. Best mileage it ever got was pulling a jet ski on flat ground at 60 mph, 26.4 high with an avg of 25.3. I made the drive from Pensacola to Atlanta on less than a quarter tank. Regens kill mileage, speed kills mileage, bigger tires and wheels kill mileage. You can make em sip, you just gotta slow down.
2016 2500 Z71 CCSB
2015 Chevrolet 155 Crew Cab nonexistent bed, Traded
1985 GMC K-1500 6.2/700R4 3.73's on 31x10.50's 95k original miles power everything.
2004 1500 Silverado 4.8 NV-3500 5-speed
1984 K10 350 4-bbl Th350 up 4 on 35x12.50
1991 C30 454 SM465
1983 K20 Suburban 6.2 700R4