As far as tonnage goes, I don't think anyone really knows where these numbers come from. My father who would now be 107 if he were still alive told me that when he got his first truck probably in the late 20's, Texas had a law that a truck could only haul 7,000 pounds, no matter what the truck weighed empty. Possibly it stems from those days. Even now when I register an tandem truck tractor in Texas, the computer asks for tonnage. For some unknown reason the magic number is 5. Otherwise the computer beeps.