........ The ST225/75R15 load range C would work, but something like the Maxxis UE-168 Bravo LT235/75R15 Load Range D aired down to 50 psi would work well too. It is 28.9" in diameter.
But if it's aired down to 50psi, it will not have the load carrying capacity that is load range D. In order to have the stated load capacity, it would require 65psi. Now, it may be true that he does not need all the load capacity that is represented at 65 psi, but as always, the rules of tire inflation still apply. Personally, I'd be more inclined to run LR D tire and inflate them to 65 psi. First make sure the wheels are rated to that inflation pressure, and I'd use metal valve stems. And as always, I would watch them with my infrared thermometer for sure for the first few trips to make sure they are not getting overly hot.
The reason I said it could be aired down is because the original tires have such a low capacity to begin with. The Maxxis tires, even aired to only 50 PSI will increase the load capacity. And as you point out, if by checking them with an infrared thermometer indicates they are running hot, there is the option to increase the inflation pressure.
got a 225-75-15 trailer tire and the hight is 28.4 needed 29 so that is ok. the withe was a little wide and the rim was 1'' offset too much, but if i have to use the spare i can take 7 screws out and remove the plastic fender thanks for all the help..