Tom,
You must not live in a state the goes both by the max axel wt of 20k, along with the max lbs per inch as the FBL's have in them. I know wadcutter in Illinois did not enforce that issue from his posts. Very common here in Wa st. You will not get the full 20K lbs unless you have 40" of tire width per axel. Thats singles any how. Duals get 600 lbs per inch, so you can go a bit narrower on the tires and still meet the 20K rule.
Then tire capacities based on speed, seen those too! My bobcat has 10.50x16.5 8 ply tires on it. Good to 4500 lbs, IF the speed is kept below 25 mph. Then the rating goes down to 3000 lbs, More like a typical LT tire of the same size load rating etc.
As many have said, best to know what your state will or will not enforce, how it will enforce it etc. RCW45 in Wa state will let you know what you have to follow here! One can also do a search for the federal bridge laws, find out what those rules state, how they can be followed etc.
Marty