By Roger Marble
I recently saw an RV forum post about better durability of trailer tires. The poster said: “Old trailer tires were rated to 65 mph. Newer tires are rated to 81 or 88 mph. That is a big difference. Since I upgraded to Load Range D and Speed Rating M (81 mph), my trailer blowouts have disappeared.”
Glad you are getting better durability from your trailer tires. I would not attribute all of the better durability to the higher speed rating. We are seeing that many/most ST tires manufactured since 2018 are more durable than the tires of 2000. The increase in Load Range probably gave your tire durability a good boost.
One thing you can look at is the material list molded on the tire sidewall. I think you will see that older tires did not list a layer of nylon or other material on top of the two steel belts in the tire tread. However, many of today’s ST tires list nylon.
We need to remember that “Speed Rating” is a bit misleading. ST-type tires have their load capacity formula based on a max operating speed of 65 mph since the 1970s. Otherwise their load capacity would be similar to LT-type tires.
How “Speed Rating” is established for tires
The test used to establish a “Speed Rating” is a 30-minute step-speed test designed for passenger-type tires. To pass a “speed level” a tire only has to be able to run 30 minutes without failing, after which the tire is scrap. So, clearly a tire with an 81 mph “rating” should not be considered acceptable for running many cumulative hours at 80 mph.
We really need to only use the “rating” as a measure of RELATIVE heat resistance. Also, it can be misleading to try and compare the rating on tires from company “A” with tires from company “B”. Each company will use test results from a small number of tires that are actually tested to establish the “rating” symbol for that group of tires. This is done statistically. The statistical prediction used by company “A” is not going to be the same as used by company “B”.
Also, there is no DOT test for this rating. So I doubt that you can depend on all tires of a specific speed rating to perform the same. With no federal regulation for speed performance, I do not see a reason to expect any ST-type tire to be capable of running at the stated speed for more than 30 minutes when brand-new on a perfectly smooth surface as used in the test laboratory.
When shopping for new tires for your trailer, I suggest you “read the fine print.” Look at the material list for the tread area molded on the tire sidewall. An ST tire with more than just steel and polyester listed, e.g., Polyester + 2 layer Steel + 1 layer Nylon, will probably be more durable than one without the nylon when operated at speeds above 65 mph. IMO a “speed rating” of L (75 mph) should be more than sufficient for RV trailer use.