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Trailer tires are designed with a stiffer sidewall to prevent swaying. I would never put a car tire on any trailer. I have seen truck axles used in trailer fabrication. The truck tire has a stiffer sidewall rating. You have a dual axle trailer for your Newport which may be an overkill. Add up the maximum tire load and multiply by 4. That will give you the load rating (must add in the weight of the trailer itself). Figure out the weight of your boat outfitted. If a decent discrepancy in your favor you may sneak out savings getting used truck tires (if you can find some that small).

You may want to convert to a single axle with an axle/tire capacity/springs for your boat. With springs, bigger is not better.

Garris