Tuesday, September 26, 2023


How do you figure an ST-type tire’s maximum safe speed?

RV Tire Safety
with RV tire expert Roger Marble

Originally posted on an online forum:
“The 65 mph is not really correct. It depends on load and air pressure. They can go to 80 mph.”

As I have pointed out previously: The load formula that populates the Load/Inflation table, used by the tire industry for ST-type tires, is based on the assumption that 65 mph would be the MAXIMUM operation speed. If the operation speed was to be between 65 and 75 mph, then users would use a 10 psi higher value for their measured load.

Example: ST235/75R15 with 2,030 lbs. measured load needs to run 50 psi minimum cold, not 40 psi cold.

If running 75 mph to 85 mph, the Load capacity number must be reduced by 10% in addition to the +10 psi adjustment.

Example: ST235/75R15 would be rated to carry (2,030 x .9) or 1,827 lbs. at 50 psi.

I know this adjustment process seems backward, but that is the way tire loading is calculated.

Now, I am sure some will say that the new ST tires come with speed ratings faster than 65 mph. IMO many of these ratings were applied just to avoid import taxes. I know of no magic rubber that somehow gives an ST235/75R15 LR-C the ability to support 2,340 lbs. at 87 mph with 50 psi in it, while an identical sized LT235/75R15 LR-C is only able to support 1,985 lbs.

Before you say, “Ya, but the tire companies probably made big improvements in the new ST tires,” I would ask what makes you think the tire companies would not want to be able to offer better load capacity to their LT tires?

Load capability is basically the tire air volume x air pressure, with adjustments for speed and expected service. So if you have a pickup pulling a travel trailer, the “service” would be the same and the speed would be the same, so how can the tire with “ST” on the sidewall carry more load at the same speed? Magic?

Now, you are more than welcome to believe in magic or marketing claims, but IMO using the load/inflation tables without doing the adjustments will probably result in an increased likelihood of belt separation. So when you have a failure please do not post something here along the lines of, “I just had a blowout. I didn’t hit anything and always check my air.”  Tire failures usually occur because of cumulative internal structural damage from heat and time. The excess heat comes from the combination of speed/inflation/load.

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


Roger Marble
Roger Marblehttp://www.RVTireSafety.net
Retired Tire Design and Forensic Engineer w/50+ years of experience. Currently has Class-C RV. Previous Truck Camper, Winny Brave, Class-C & 23'TT. Also towed race car w/ 23' open trailer and in 26' Closed trailer. While racing he set lap records at 6 different tracks racing from Lime Rock CT to Riverside CA and Daytona to Mosport Canada. Gives RV and Genealogy Seminars for FMCA across the USA. Taught vehicle handling to local Police Depts



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