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The max tire pressure is required for the tire when running the max load rating for that tire. If you're running less weight, then less pressure is needed. If you do run max pressure without running max load, the tires will wear unevenly in the center of the tire.

My whaler tires are 8ply and rated for 100psi. I run them at 55psi because the 13' whaler isn't that heavy. For our other boat, it's a tandem trailer and the boat is very heavy so I run those tires close to the max pressure.