By Roger Marble
As an actual Tire Design Engineer, I can assure you that there is more misinformation or partially correct information out there on the internet than technically accurate information regarding tires.
It is true that the original tire selection is the responsibility of the RV manufacturer. The issue is that once the RV is sold it seems that most RV manufacturers have little or no interest in standing behind their choices with any actual warranty service when it comes to tires.
It seems that OE (original equipment) tire selection for most RVs is based on one goal: Find the smallest, lowest-cost tire that will meet the requirements.
The only Federal (DOT) requirement is that the tire load capacity, times the number of tires on the axle, be AT LEAST equal to the maximum load rating of the axle. While RVIA (RV Industry Association) now requires 10% Reserve Load capacity, DOT does not. As a point of reference, most cars come with a 20% to 30% or higher Reserve Load capacity
A smaller tire can mean the RV manufacturer can get away with a less costly (i.e., smaller) wheel and maybe a smaller wheel well. So this is extra pressure on the purchasing department to get the minimum possible tire that can meet the requirements.
Given the above, it is up to you, the owner, to decide if you want any, some or more “Reserve Load capacity” for your RV. You may have the option of larger tires. Or you may be restricted to trying to find tires of the same dimensions but with higher load capacity.
Types of tires
You need to educate yourself about the requirements and limitations of the four “types” of tires that are in the market.
P-type tires
‘P” is Passenger type. If used on an RV (trailer or motorhome), the load capacity must be reduced by dividing by 1.1. But not everyone will know or do that.
LT-type tires
“LT” (Light Truck) type tires can be used in RV service. However, you will soon discover that LT tires with the same dimensions and Load Range (ply rating) have a lower load capacity than the same dimension as ST type.
ST-type tires
“ST” (Special Trailer) type tires have the highest load capacity rating for a given set of dimensions. But you need to remember that the ST tire load formula that is used to calculate the tire load capacity is based on an assumption of a 65 mph maximum speed. We all know that there is “No Free Lunch” and the trade-off for increased load capacity is lower speed capability. The “Speed Rating” symbol on many ST type tires is based on a 30-minute test. So you need to decide if you want to depend on such a short-term test when making a tire selection.
Truck/Bus tires
Finally, there are actual “Truck/Bus” tires. These have no leading letter and are usually on 17.5″ or larger wheels. These tires have higher Load Range, usually F or higher. These tires are almost all rated for 75 mph in RV use on the highway.
Do your homework. Ask questions, but remember there are very few really knowledgeable people out there who have the training or experience in tire engineering. Just having driven on tires for 40 years is not the same as having been held responsible for designing tires for Truck, Passenger, Trailer, or Indianapolis racing application. Also, being able to read Federal Regulations is not the same as having to work within those regulations while meeting the goals and demands from GM, Mazda, Toyota, Honda, Freightliner, MB, Nissan, Ford, or Chrysler.
Incidentally, I am only aware of two actual Tire Design Engineers who regularly post on various RV Forums.
Have a tire question? Sign up for Roger Marble’s new Facebook Group: RV tire news, information and discussion, 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.
##RVT1000
Thanks for the excellent description of the various tire types. Happy with the current tire wheel size on the RV now, but would consider going to a larger combination when time (4 years from now).
Thanks for the article, Roger. I have a question about your last statement under ST type tires.
I have to ‘decide’ if I want to trust the 30 minute tested rating? Well, if that is how the manufacturer tests them, just what choice do I have?
I worked in a computer engineering environment for many years. Like most engineers, you and I often suffer from the tendency to point out the drawbacks in each option available, without helping to bring others to the best possible conclusion/decision. After reading your comments on each tire type, one might conclude it is best to just drive on the rims.
I don’t see where you actually answered the title question – but assume the answer would be “yes”. Maybe a follow-up article on how one would go about determining the optimal wheel/tire size solution? I have a TT with 14″ wheels, was considering upgrading to 15″ wheels. What benefits would this really provide? Greater load capacity? Higher speed rating? Cooler running temps? Are there any downsides to such a move?