We have exactly what you're looking for! I recommend going with the Karrier ST225/75R15 Radial Trailer Tire - Load Range E # AM10303 which also has that 81 mph Speed Rating M you're needing. You can then pair this with a steel wheel like the Taskmaster Steel Modular Trailer Wheel # 560545WM1 which has the higher weight rating to match this tire and the 5 on 4-1/2" bolt pattern you're needing.
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