Quote:
Originally Posted by Montana Man
Calandlinda, it seems you know a lot about tires. Is there some regulation you can post? I think most of us are aware about load ratings and such but if there are regs on changing sizes of tires while equaling or better the load rating, it would be helpful.
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The answers you seek is not in any single regulation. It's within the safety standards of the industry.
Without going into thousands of words that would have to be written to combine the safety regulations involved with validating my statements I'll just provide a short answer.
Even though it's not practiced in the application of RV trailer tires it is still provided in the vehicle owner manuals. "When seeking replacement tires look on the vehicle certification label for the correct size or get approval from the vehicle manufacturer for another size". By not doing that you violate a safety standard.
The proper nomenclature for a tire size is
designated size. That includes the prefix, such as ST235/85R16.
When a consumer uses a replacement tire that is not on the vehicle manufacturer's approval list, such as using a ST235/85R16 to replace a ST235/80R16 you invalidate the vehicle certification label because the designated size differs. Which in the tire industry is called a misapplication.
Everything changes when the vehicle manufacturer agrees to the larger tire size. NHTSA allows the use of an axillary tire placard describing the new designated size and recommended cold inflation pressure. The axillary placard is placed adjacent to the original placard.
Load ranges and tire speed letters are not part of a designated size. Designated sizes are identical, except in internal construction, which allows the higher load range to be inflated to an increased inflation pressure to obtain more load capacity. For instance; The ST225/75R15 LRC and LRE provide identical load capacities at 50 PSI.
The USTMA writes tire industry standards in support of the standards vehicle manufacturers must follow when building your trailer. This is a verbatim quote about replacement tires. "Replacement tires should be the same as the OE size
designation, or approved options, as recommended by the vehicle or tire manufacturer".
Bottom line: The vehicle manufacturer set a minimum Original Equipment tire standard when they did final certification. Within the standards, the vehicle manufacturer must comply with a statement that directs them to provide tires that are appropriate for that vehicle and set a recommended cold inflation pressure for them to provide load capacities sufficient for the certified GAWRs.
It's been my observation that RV trailer manufacturers will not approve any replacement tires that do not meet the appropriate sized they provided at vehicle certification, unless they had offered, prior to delivery optional designated sizes.
References for this post have come from FMVSS (standards), 49 CFR vehicle certification, Keystone generic owner manual and the USTMA.