Originally Posted by jlgail4309
Is that a speed rated to 75 miles per hour.
Goodyear Marathons and other ST tires (I would bet) will benefit from the information in the attachment from Goodyear.
Basically, ST tires are speed rated at 65 MPH; BUT if you have the excess load capacity, you can adjust the inflation pressure upwards by up to 10 PSI over the required pressure for your load (also within the capability of the rims) to add up to 10 MPH to the speed rating.
For example, if your camper came with MAXXIS ST225/75R15 C rated tires and when you hit the scale your camper's load on each tire was 2000 pounds. At 2000 pounds of load (according to the Maxxis chart) your "correct 100% inflation pressure" would be 45 PSI and not the maximum inflation pressure of 50 PSI.
In the above scenario, running your MAXXIS ST225/75R15 C rated tires at 50 PSI at the 2000 pound load would increase your sped rating to 70 MPH.
It would be impossible to increase the speed rating above 70 MPH in this case without up grading your tire range to "D" in order to use a higher than C rated inflation pressure of 55 PSI. If you bought 4 of the D rated MAXXIS ST225/75R15 tires, and ran the pressure at 55 PSI with a 2000 pound loading you would achieve the best case speed rating of 75 MPH. (A 10 MPH boost for every PSI above that required to carry the load.
This only works if you have the load and pressure data for your tires AND KNOW the actual load on your camper's tires (get weighed and weigh each axle's load; not both and divide by 4 as the rear axle may be carrying more of the camper's weight than the front if the camper is not exactly level when connected).
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