Tonnage ratings have been meaningless as long as I can remember and I have been messing with trucks since the 1950s and 40's vehicles were still common then. My father started in the gasoline business in the 1930s. He talked of a time when a truck could only haul 7,000 lbs payload with no regard to gross weight. This law must have changed in the early 30's and I think this is were these nominial tonnage ratings originated. When I purchase a truck tractor from out of state, Texas DMV asks for tonnage. The system will only take 5 tons even though the gross weight will be 80,000. I think the tonnage rating just stuck and no one knows why.
I use Google+ and post links to pictures. Here are a few of my old farm trucks.
https://plus.google.com/u/1/photos/photo/112636930370126476644/6191993961092761426
