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Is it illegal to change tire type or size on an RV?

There seems to be a recurring question of whether or not you can change the brand or size or load range of tires on an RV. There are a few individuals who have interpreted the Federal Regulations on tires that apply to the vehicle manufacturer and they want to apply the same regulations to the current owner of the RV.

Since I had to live under Federal Regulations for almost four decades, as I developed and tested many different types and sizes of tires, I do not agree with the idea that owners are prohibited from making changes—with one basic exception. The load capacity of the new tires must be equal to or greater than the load capacity of the original tires.

I have seen 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 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 or greater.”
  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.
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.

Have a tire question? Ask Roger on his new RV Tires Forum here. It’s hosted by RVtravel.com and moderated by Roger. He’ll be happy to help you.

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

 ##RVT1086

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Thomas D
22 days ago

Are tires in tandem like most rv’s considered dual for rating purposes or do they have to be side by side to be a dual arrangement? You see single and dual rating on tires.

captain gort
22 days ago

I increased the tire size on my Forest River TT. I bumped them up 1 size and gained aboout 200# capacity on each of the 4 tires. Now, that does not mean I will load the trailer any heavier than it is rated for on its door sticker….but it gives me peace of mind knowing I have a bit more safety margin. And yes, the rim width was still in the the proper range for the larger tire.

Gary Broughton
22 days ago

After the 16 inch cheap crappy tires on my 5er went away I put the same tires on that my truck ran. Firestone 500s at 50 psi only.

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