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?