My 7x23 all aluminum sled trailer came with GY Marathon 14" with load C. The GVWR is 6K with 3,800 payload. I've towed 70+ mph many times, on long trips, loaded with 3 MT sleds, lots of gear, tools, and extra fuel.
My trailer has the rubber torsion suspension, and rides, tows very well, even loaded heavy at speed. I don't think you need more than C-rated tires, that would ride smoother, less movement of things inside trailer. Maybe can't get the Endurance in a C?
I would look at the Provider STs, the Carlisle HD, and consider the Endurance. All are new style, cooler running, higher speed rated, 81-87 mph.
Here is a pic of my trailer.
Jerry
