Recently I was asked to clarify some information an RV owner discovered as he did some research. What we see here is an error that is easy to make. Even for the more experienced RV owner.
The RV owner asked about a Michelin XTA
215/75R17.5 LR-J tire that came on a 5th wheel trailer. He had confirmed the load capacity of the tires at their max rated inflation and said they could handle the load on his 8,000# axles when inflated to the placard inflation of 120 psi. His question was about the max speed rating of the tires.
He consulted the Michelin Truck Tire Data Book and found that the subject tires were intended for Low Platform Trailer application.
In fact the brochure for his tires even identified the maximum speed rating for his tires as 62 mph.
He wanted to know if the maximum speed rating had been waved for RV application, and if it had what was the new speed limit. He found a statement from the Tire & Rim Association that specifically stating that when a tire had a speed rating below 65 mph it could not be manipulated (increased) with adjustments in inflation and load. He also found a Michelin truck tire service manual specifically saying that low bed trailer tires like the XTA, cannot have their speed increased.
Despite all this information he wanted to know how to work out the adjustments needed to allow a higher speed limit for the RV and could I please help him understand what he felt was confusing information given that the RV manufacturer had selected these tires for his trailer.
I had to tell him that in my opinion there was no way the speed limit could be adjusted. I pointed out that his error was in looking for information in data books and manuals of different type tires such as Truck Tires, RV tires and Low Platform trailer tires and assuming that since his tires were sort of like truck trailer tires and looked like tires he has seen on other large 5th wheel trailers, he could pick and choose which information applied to his specific tires.
The lessons to learn here are:
1. Not all RV manufacturers pay a lot of attention to proper tire selection for the application.
2. When doing research on tire type and specific operation limitations you need to look only at the literature for your specific tire if such literature is available from your tire manufacturer.
3. You cannot depend on the RV dealership to warn you about the real quality or limits of some components of your RV. You need to take responsibility for what you purchase. i.e. Let the buyer be aware.
Speed ratings are like the “Red Line” for your car, truck or motorhome engine. While it is possible to exceed the max speed rating of a tire just as it is possible to exceed the rev limit and run your engine in the red zone, I think we all understand that if we do run our engine that fast we are pushing the odds. In the same manner if your tires are rated for 65 mph max you might be able to run faster for a short time but you are consuming the finite limit of the tire structure and simply slowing down does not repair the microscopic damage you have done to the internal structure of the tire. You should not be surprised if you blow your tire just as you would not be surprised if you blow your engine after running in the red zone for engine speed.