Originally Posted by herk7769
The max is "up to" 10 PSI over the "pressure required for the actual tire load" taken from the charts.
If the tire is already at its max load at its max sidewall pressure, no further increase in pressure is allowed and speed rating is then 65 MPH.
Where did the statement "max is "up to" 10 PSI over the "pressure required for the actual tire load" taken from the charts
" come from?
Your statement "If the tire is already at its max load at its max sidewall pressure, no further increase in pressure is allowed and speed rating is then 65 MPH
" is not true for GY Marathon tires per the GY Tech bulletin.
The point of my doing this thread was to try and clear up the question of the max cold inflation pressure.
1. The pressure molded on the sidewall that is associated with the max load is often incorrectly called the "max pressure". This pressure is just the cold inflation pressure associated with the max load which is molded on the sidewall.
2. The normal max speed for ST type tires is 65mph unless:
A. You have a published document from the RV MFG stating the specific tires applied by the RV MFG are rated for a different max speed. This might include alternate load and inflation numbers. This would ONLY apply to the exact Brand, size, line and Load Range of tires supplied by the RV MFG.
B. You have a published document such as the example Goodyear Tech Bulletin with alternate specifications.
3. In the case of Goodyear Marathon tires, Goodyear is allowing a max speed of up to 75 mph only if the original inflation is increased by 10psi. The original inflation would be the inflation on the RV Tire Placard.
4. I contacted GY engineers and was told I could get clarification from TRA Engineers. The TRA Engineer confirmed that it was allowed to increase the tire inflation to 10psi above the inflation molded on the tire sidewall. (see#1 above)
5. None of the above inflation increases allow an increase in load capacity beyond what is published in the Load Infl tables for the original inflation.
If you have a GY Marathon ST225/75R15 LR-D it is rated for 2540# @ 65psi and a max speed of 65 mph. If you increase the cold inflation to 75psi the tire would now be rated for 2540# @ 75psi and a max speed of 75 mph.
The example currently only is OK for Goodyear Marathon ST tires as I have not seen a published document from any other tire manufacturer.