I have been looking for a better quality tire for my 5th wheel and it seems that i'm limited by my 15 inch rims. I'm currently running a 225/75-15 tire and the wheels are rated at 60psi max. Who sells replacement rims for 5th wheels in a 16 inch size with a greater working pressure? My axles are 6 lug.
Make sure that you've got enough clearance between and above the tires when changing tire sizes. If you stay with 15" tires, the only other real alternative is to go to Maxxis E-range. Just be sure that your wheels will take the 80 psi that an E-range tire takes. FWIW, if you have D-range tires now, they should be 65 psi max cold pressure. If it boils down to money, and you stick with a D-range tire, Maxxis is still an excellent alternative.
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You say your current rims are limited to 60 psi. That would indicate to me your current tires are load range C. These would be rated 2150 pounds each. An ideal replacement would add an additional 15% load margin. That would be 2530 pounds each. If your rims would handle 65 psi, you could have done that with the load range D Maxxis. But, given you need new rims anyway, take a look at the BF Goodrich Commercial TA in LT225/75R16 load range E. This tire is about 1.5" larger in diameter than your current tire and has a load capacity of 2680 pounds at 80 psi. They will make your trailer ride 3/4" higher and take up 3/4" more room in your wheel wells (try to maintain at least 3" in your wheel wells). They will also reduce the space between your tires by 1.5". You need to maintain at least 1" there. The LTs in this size will be 0.1 inches wider than your current wheels, so they would end up 0.05 inches closer to the frame. This is negligible.
The BF Goodrich Commercial TA is a popular replacement with a tread pattern that lends itself well to trailer service. I and many others here on rv.net use them on our trailers.