Is it OK to run 70+ mph with ST-type tires? It depends …

RV Tire Safety
with RV tire expert Roger Marble

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

I’ve read more posts on an RV trailer forum about driving at 70 to 80 or more. Here is my reply.

The load formula that resulted in the numbers in the Load & Inflation charts has not changed since the late ’60s. That formula was based on a 65 mph MAXIMUM operating speed. The lower speed was the trade-off you get when you increase the load capacity. “There is no free lunch.

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

The “Speed Symbol” is simply an indication of temperature resistance of a tire on a short-term (30 min.) step speed test. The symbol is based on an SAE test that specifically is for Passenger-type tires. ST type is not covered by SAE.

Those who choose to run at 65+ 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 a Science, folks – not magic.

So please, when you have a belt separation failure 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 decided to not heed the warning and advice.

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

IMO, the addition of higher 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 2017, and, 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.

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:

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 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 for 30 minutes, then at 400 rpm for 30 minutes, and then at 425 rpm 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.”

The RPM converts to 50 mph, 75 mph, 80 mph and 85 mph, so the 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.

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

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 how 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 same better construction features they apparently are putting in the ST tires in the LT tires, where load capacity is an important factor in selecting the vehicle?

 ##RVT866

 

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11 Thoughts to “Is it OK to run 70+ mph with ST-type tires? It depends …”

  1. John

    I have Goodyear endurance tires installed on my travel trailer. They are rated over 75mph. I normally travel at 65mph but I do occasionally run up to 70mph. I stop every few hours just to get out and stretch. I carry a infrared thermometer and always check my tires and wheel bearings. My trailer tires will be about 110 degrees and my truck tires were 120 degrees. The tires hot days and cool days running 65 to 70 mph so I feel safe running up to 70 if the wind is not high and there is little traffic.

  2. Seann

    Stopping a freight train at 80 mph (that is what you are pulling an RV) is not a quick job and those that make excuses for why they have to go that fast are putting their families lives at risk.

    1. Gary

      Not just their families….20 other travelers too. Just stupid to drive that fast.

  3. Wane

    I recently had to replace a tire and the shop convinced me to use an LT Hercules tire as he showed me how much stiffer the 14 ply side wall was compared to the 14 ply st tire I had that was finished long before it should have been.
    He explained how this Hercules lt tire is now commonly installed on industrial trailers, car haulers etc.

  4. Wayne

    I want tires that are safe to pull at higher speeds regardless.
    Most of our pulling is done on 2 lane highways and I think it improper to hold up traffic just cause I would like to get better mileage and am in no hurry. That is dangerous too.
    I’ve often said I am going to put a decal on the back of my trailer that says ” Sorry I Have To Drive So Close in Front of You”!

    1. Gary

      If they wanted to get there quicker, they shouls have started earlier

  5. Tommy Molnar

    70-80 mph towing an RV trailer just scares the bajesus out of me. Most I do is 60, and many times I just putt along at 55. Good for fuel mileage, good for the longevity of tires, and our trailer in general. But then, I’m retired and have more time than non-retired folks. I guess . . .

    1. RV Staff

      You’re exactly correct (and very smart), Tommy. And that’s why it says it’s OK to drive that fast only “if you believe in magic.” 😯 —Diane at RVtravel.com

    2. Jeffrey Torsrud

      You are correct. Being Safe and driving at slower speeds is just Smart.

      I don’t know understand why anyone wants to drive down the road at more than 70 MPH??? I mean come on! Yet, I see it all the time when driving down the Highway, my Max speed is always between 60 and 63 mph, never faster!

      Those who read these replies, please SLOW DOWN!

      1. Gary

        I think NTSB records will show that someone driving that fast only arrives at their destination anout 150 to 300 feet ahead of the slower safer drivers anyway. The roads we have to drive over are not Alpine road race rated courses

    3. Gary

      You didn’t even mention that 55 and 60 is just safer!!!!

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