Originally Posted by Rickflint
Are either of these tires rated for speeds higher than 65? Looking for something rated for 70.
All ST tires are speed rated for 65 MPH AT THE RATED LOAD.
If you have "room" to add pressure to the tire (not to exceed its max rating) you can add 1 MPH for every PSI above the correct pressure required for the load (up to 10 PSI maximum for 75 MPH possible). See attached product data sheet and Maxxis (as an example) for the following explanation.
Say you have ST225/75R15 Load range "C" tires and when you weighted the camper the actual load on each tire was 1880 pounds. With this load, those tires require 40 PSI (50 PSI max for load range C) to support that load and have a speed rating of 65 MPH.
Increasing the pressure by 5 PSI over the required pressure of 40 PSI (45 PSI in the tire), you will increase the speed rating to 70 MPH. Increasing to 50 PSI (the max allowed for these tires), you will increase the speed rating to 75 MPH. NOTE: This ONLY works if you have 1880 pounds (OR LESS) of load on the tire!
If the next time you go out and weigh the camper, you discover you have 2020 pounds of load on the tire; you will need to have 45 PSI in the tire to have the correct tire pressure for your load and the speed rating will be back to 65 MPH. You could add up to 5 PSI (again you can not exceed the sidewall max) to boost your speed rating to 70 MPH; but no higher.
If your camper's tires are maxed out at 2150 pounds per tire (50 PSI required), you are screwed and are stuck with the 65 MPH rating since you can no longer increase the pressure. A solution is to upgrade your tires (keeping the same size tire) to load range "D" (65 PSI max) allowing you to run 10 PSI higher (60 PSI - provided your rims and stems can support that pressure) and run at 75 MPH speed rating again.
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