someone ask for a 205/ -15" D rated tire. Their out there. See maxxis tire web
http://http://www.maxxis.com/AutomobileLight-Truck/Light-Truck-SUV/UE-168N-Bravo-Series.aspx for UE-168 LT tires.
The trailer is weighed per axle and tire capacities per NHTSA/DOT571.110/.120
S4.2.2.1 Except as provided in
S4.2.2.2, the sum of the maximum load
ratings of the tires fitted to an axle
shall not be less than the GAWR of the
axle system as specified on the vehicleç—´
certification label required by 49
CFR part 567. If the certification label
shows more than one GAWR for the
axle system, the sum shall be not less
than the GAWR corresponding to the
size designation of the tires fitted to
the axle.( end of quote)
Dot weighs the axle loads on the trailer and then looks at the tires for matching numbers. If there is a difference then a DOT officer may go under the trailer to see what the axle are tagged at. Tires capacities and axle ratings had better = GVWR. Some state DOTs go to the tires capacities. Some go with axles and tires.
The only place the truck or trailers GVWR is used is for a combined rating. The only place a combined rating is used is on the commercial side for over 26001 combined towing.
The 4x4 tractor/loader/mower in the OP thread will probably be to heavy for a trailer with two 3500 axles. The tractor probably weighs over 4000 lbs plus another 500-600 for the mower and another 1500 for the loader plus fluid filled tires.