How fast can you safely travel with ST-type tires?

Is it OK to run 70 or 80 mph or faster with ST-type tires? Yes—If you believe in MAGIC!

I continue to read posts on various RV trailer forums about driving at 70 to 80 mph or more with their RV trailer equipped with ST-type tires. Here are my thoughts.

The basic load formula that resulted in the numbers in the Load & Inflation charts has not changed since the 1960s. That formula was based on a stated 65 mph MAXIMUM operating speed. The lower speed stated for ST-type tires was the trade-off you got when you increased the load capacity above similar tires like Light Truck type. “There is no free lunch.”

Look at the load capacity of similar-sized LT tires. If companies that make ST tires suddenly discovered some magic rubber or special tire design feature, that did not require some tradeoff, why don’t they put that magic stuff in their LT and Passenger tires?

Speed Symbol

The “Speed Symbol” is simply an indication of high temperature resistance of a tire on a short-term (30 min.) step speed test. The symbol is based on an SAE test that is specifically for Passenger-type tires. In my opinion, it is used as a marketing tool. ST-type tires are not covered by SAE.

Those who choose to run at 65 mph and higher need to remember that you are “consuming” the finite number of cycles that make up a tire life. Faster speed = higher temperature. Higher temperature = shorter life.

This is science, folks… not magic.

So please, when you have a belt separation failure aka “blowout” before you wear out your tires, do not come back here with some complaint about “defective” tires or “China bomb” tires. You were warned, but you decided to not heed the warning and advice that is derived from basic tire engineering.

The high-load capacity in ST tires was developed based on a stated 65 mph maximum speed.

Speed ratings

IMO, the addition of speed ratings was not accomplished with improved engineering but because of import tariffs being imposed on non-speed-rated tires by the U.S. Department of Commerce in late 2017. Then, as if by magic, each and every tire company making ST-type tires almost overnight added high-speed capability to the tires. There were no years of incremental improvements in speed capability. If that were the case, we should have seen 70 mph tires, then 75, etc.—but we didn’t. I know this from firsthand experience of developing “H”-speed rated Passenger tires in the early ’80s.

I can say that based on my examination of tires in the Goodyear “Endurance” line, they did make improvements over the previous “Marathon” line. Was it enough to justify an increase in speed capability? Or was it just enough to improve upon the unacceptable short life, as reported, on Marathon ST tires? I do not have enough data to be able to answer that question. But based on anecdotal reports from users, it appears the Endurance line can deliver acceptable performance in most RV usage as long as speed is kept under control.

Some have suggested the increased speed capabilities are the result of years of incremental improvements and that tire companies have seen numerous slight improvements. I understand this concept. The problem is I don’t know how to exceed a “Step Speed” test and translate a non-specified step into a speed capability that is not part of the test.

Here are the actual test requirements:

Department of Transportation (DOT), Federal Motor Vehicle Safety Standards (FMVSS) High-Speed test says:

S7.4 High speed performance.

(a) Perform steps (a) through (c) of S7.2.

(b) Apply a force of 88 percent of the maximum load rating marked on the tire (use single maximum load value when the tire is marked with both single and dual maximum loads), and rotate the test wheel at 250 rpm (50 mph) for 2 hours.

(c) Remove the load, allow the tire to cool to 35° C (95° F), and then adjust the pressure to that marked on the tire for single tire use.

(d) Reapply the same load, and without interruption or readjustment of inflation pressure, rotate the test wheel at 375 rpm (75 mph) for 30 minutes, then at 400 rpm (80 mph) for 30 minutes, and then at 425 rpm (85 mph) for 30 minutes.

(e) Immediately after running the tire the required time, measure the tire inflation pressure. Remove the tire from the model rim assembly, and inspect the tire.

Note the load reduction to 88%. Were you ever told to reduce the load on your ST-type tire? Note the speed steps are not linear. What would you consider exceeding the specified test by 15% looks like? Run 34.5 minutes at 85, or run another 30-minute step at 97.75 mph?

The Speed Test above is required for a variety of tires, with ST being one type. LT and Passenger tires have different standards and different test requirements.

DOT doesn’t recognize speed symbol system

Note that none of the DOT tests support any “Speed Symbol” claim, as DOT doesn’t recognize the speed symbol system.

The SAE high-speed test is not a regulatory test. “This SAE Recommended Practice provides a method for testing the speed performance of passenger car tires under controlled conditions in the laboratory on a test wheel. This procedure applies to ‘standard load,’ ‘extra load,’ and ‘T-type high-pressure temporary-use spare’ passenger tires.”

Maybe someone can explain why a P235/75R15 would be rated for 1,874# @ 32 psi and can carry an “S” speed symbol (112), but in trailer or pickup service is only rated to carry 1,703#.

An LT225/75R15 is rated for 1,445# @ 35 psi and can carry an “S” speed symbol.

Then we see an ST225/75R15 is rated for 1,760# at 35 psi with a Speed Symbol of “N” (87).

If you only want to look at dimensions, load and inflation, I don’t know why all three wouldn’t be rated for 1,703# to 1,760# load capacity.

Why haven’t the tire companies put the better construction features they apparently are putting in the ST tires in the LT tires, where load capacity is a primary factor in selecting the vehicle?

My observation

This Max speed is like your engine red-line. While it might be possible to exceed this speed for short periods, it will significantly reduce long-term durability.

Roger Marble

If you have tire questions, check out Roger’s Blog as well as his posts on RVtravel.com. There are hundreds of posts covering everything to do with tires.

If you still have a question for Roger after searching the above posts, send your inquiries to him using the form below.

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