have 2 '05 2500HD 4x4, 1 c cab 6' box & 1 ex cab 8' box. both getting 17-18 intown & 21-22 on out of town trips around 60 mph. about 1.5 mpg less per 10 mph faster. the fact is you have to decide how fast vs how much fuel to use. no free lunch, remember.