I don't think that you'll beat 16 mpg with any vintage inline 6 cylinder engine driving in the modern environment. Lots of combos exist as to tire sizes, overdrives, power levels, etc, and I don't think I have ever seen anyone claim more than 16 mpg that wasn't exaggerating. The aerodynamics, or lack thereof, of these trucks at higher speeds (Above 50mph) just doesn't lend themselves to a good mileage result. And then when you do get a nice powerful engine and overdrive, it's just too much fun to keep your foot out of it!
