with RV tire expert Roger Marble
I have been following a series of posts on RV forums where people ask about changing tires (size, type, load range or cold inflation setting). Occasionally I run across some people who have adopted a hobby of commenting on tires in RV application, and with little working knowledge, who make pronouncements on the “legality” of making any change in tires that I do not agree with.
Other times I see a question like this one:
“My research (curiosity vs. need) is that LT and ST tires are not sold in the same sizes, so changing RIMS would be required?”
Here is my reply:
Some LT tires show the same “dimensions” (for example, 235/75R15) as some ST tires but the “dimensions” are NOT the actual complete size “description,” which includes the letters and numbers before and after the dimensions, i.e., ST235/75R15 110/105L LR-C versus LT235/75R15 110/107T LR-D. In this example, the ST tire is rated for 2,340# @ 50 psi, while the LT tire needs 65 psi to support 2,335# (single load capacity shown). Also the ST tire is rated for a max of 75 mph while the LT is rated for 118 mph operational speed.
Yes, there are some ST tires where the dimensions do not match any tire identified as an LT type currently on sale. This is where you have to do some research and learn some facts on what can be considered a reasonable and safe change in tire type or size.
The key items to confirm:
1. Is the Load Capacity of the new tire equal to or greater than the original tire when you consider your new intended cold tire inflation? This does not mean +/- 25#. It means “equal to or greater than.”
2. Is the Speed Rating equal to or better than the original tire when you consider your intended new cold inflation level? (Yes, some tire load capacities are a function of speed.)
3. Are your wheels rated for the inflation level you intend to run with your new tires? This limit may not be easy to learn, but wheels can fail from too high a pressure just as they can fail from too many pounds load. The pressure we are talking about here is always the COLD inflation pressure.
You may need to be smarter than the tire guy if you want or have to make a change.
Remember, there are some who post on various RV forums who would tell you that you would be violating Federal Regulations if you change from a Goodyear Marathon made in 2016 to any Goodyear tire made in 2019.
While it is definitely true that some people make changes in tire “dimensions” or “type” or inflation level – IMO, as an actual tire design engineer, I would consider those changes unwise. BUT that does not mean you cannot consider making changes as long as you follow the guidelines posted above that follow the published guidelines from major tire companies.
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