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Friday, March 6, 2020

Is it against federal regulations to change tires on an RV?

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 a little working knowledge, 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  vs  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 tp 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 118mph 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 or greater than to original tire when you consider your new intended cold tire inflation? This does not mean +/- 25#. It means "equal or greater"
2. Is the Speed Rating equal 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 which IMO as an actual tire design engineer, I would consider unwise. BUT that does not mean you can not 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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