ST tire speed rating, and why you should stay under 65 mph

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RV Tire Safety

with RV tire expert Roger Marble

Point of clarification. The Tire and 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- nor LT-type 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 “derating” of load capacity when used on a trailer or pickup.


If you look at similar physical-sized tires and include the P derating you will see the derated 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 tire 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- to 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 a higher speed symbol.” What I am saying is that you should still stay to 65 mph max or you may end up paying the consequences with shorter-than-expected tire life.

But that’s just my opinion.

 

Read more from Roger Marble on his blog at RVtiresafety.net.

 ##RVT894

 

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Gary Stone

My rule of thumb is: the bad things that can happen at 55 mph are exponentially worse at 70 mph.

Joe Simon

If people want to drive 70 or 75, more power to them I will wave to them as I go by at 63 mph with no blowouts, some people just don’t get it. I’m retired and we have a bumper sticker says I’m retired go around me, getting there safe is what’s it’s all about.

John McConnell

Roger what about the Goodyear Endurance tires. Rated for full load at 80psi and max speed rating of 85. Not that I would ever do 85 mph towing a trailer. The Goodyear is made in US so there would be no tariffs.

Mike Sherman

Roger knows rubber, and I’ve learned plenty over the years. For safety reasons I seldom exceed 50 mph. Tires run cooler and last longer. Besides, towing (in California) mandates a maximum speed of 55. We always enjoy the view and we arrive more relaxed. Occasionally I’ll pump it up to 60 if I have folks stacking up behind me, and I’ll use a turn out if only one vehicle is behind me.