Originally Posted by ependydad
Greenball (I think) ST tires are load rated at 75mph.
When the tire pressure required for the load is increased (up to 10 PSI above the pressure required for that load AND up to no higher than the sidewall posted pressure) to add 1 MPH of speed rating per PSI to a max of 10 MPH (75).
So if the load on YOUR tire is 1880 pounds and you are running 50 PSI max load range C ST225/75R15
tires, this load requires 40 PSI to be speed rated at 65 MPH. By running the pressure up to 50 PSI max, your speed rating becomes 75 MPH.
If those same tires have an actual load of 2020 pounds, the required pressure is 45 PSI to be speed rated at 65 MPH. Since you can only add 5 PSI to get to the max pressure of 50 PSI, the maximum speed rating at 50 PSI would be 70 MPH.
If your tire was maxed out at 2150 pounds, there can be no additional air pressure added. So speed rating in this case would be topped out at 65 MPH.
Going UP a load range to D ( 65 PSI max) will allow your 2150 pound load the "room" to add 10 PSI so you can run 60 PSI and get that 75 MPH speed rating back. (NOTE: running at 65 PSI WILL NOT get you 80 MPH! just 75 MPH).