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Friday, April 26, 2019

ST tire speed rating

Point of clarification.

The Tire & Rim Association Load formula used by the industry for ST type tires is based on a 65 mph max operating speed. They even identified inflation increase and load capability reductions necessary to operate up to 75 mph.  Neither P or LT tires have stated max speed associated with their load formula or calculation as far as I can find. P-type do have the stated 1.10 "De-Rating" of load capacity when used on a trailer or P/U.

If you look at similar physical sized tires and include the P Derating you will see the de-rated P and LT tires give similar results for load capacity.

BUT the load capacity of the ST type is significantly higher. Why would that be if there were not some other limiting factor such as the 65 mph speed limit?
Some people would have us believe that ST tires all had some magic rubber or body cord added to them in 2017 when speed ratings higher than 65 were added. The reality is that the speed rating was added to avoid import tariffs. Maybe you think that all the different tire companies used this "magic" rubber to give them an overnight increase in speed capability.
Why isn't there a single tire company anywhere in the world using this "pixie-dust" in their LT or heavy truck tires to give them an extra 10% to 25% more load capacity?

I have seen a section of a GY Endurance and it clearly has "better" construction than some non-speed rated tires. Only time will tell if this new construction results in a better life, But I seriously doubt that you can go from 65 mph max to 99 or 106 mph with an ST tire and not suffer some negative consequences.   Yes, many will say they do not drive that fast but we have all been passed by trailers going faster than 75. Some even say they tow at 80+.

Going faster can do structural damage and we have covered how damage is cumulative.
What I have not seen is anyone changing their advice on tire life in RV usage.
What ST tire company now says up to 10 years life or even claims over 5-year life?  I have reviewed a few different owner's manuals and see 3 - 4-year life suggested.
I have previously written about Interply Shear and while the addition of a Nylon cap on top of the belts may extend the life of a tire and it may also improve the tire's ability to pass some speed test designed for passenger tires I still have serious reservations about any trailer being towed at any speed above 65 mph.

I'm not saying "Don't buy tires with higher speed symbol" What I am saying is that you should still stay to 65 mph max or you may end up paying the consiquences with shorter than expected tire life.

But that's just my opinion.
 

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