While some know all of this I continue to see posts on various RV Forums where it appears some still do not understand tire markings.
It would be nice if we could all simply depend on the tire salesperson to know exactly what we need for our RV. However, with numerous tire companies making hundreds of tire designs in countless combinations of type, load range, size, and tread design, that’s an almost impossible task. With a little effort on our part, we can help the salesperson offer a smaller, more reasonable selection and end up getting a set of tires that better fits our individual needs.
What brought this to mind was a question from a reader. They said, “I got new tires yesterday, but as I was looking through my records they have a different number vs. my old tires. The new ones are P235 /75 R15 108S XL WW and the old ones are P235 /75 R15 105S OWL. So why is it different and will it be a problem?”
The salesperson may make a mistake or just be trying to sell the tires they have in stock.
While the above question is about passenger car tires, I think we can use it as a learning example.
Translating tire information
First, let me help everyone translate that tire information and nomenclature.
The “P” stands for Passenger. They’re usually used on our regular cars but sometimes used on smaller lightweight trailers. Most trailers and 5th wheel RVs have “ST” for the type. Truck tires would normally start with “LT”. This type is found on pickups, Class B and Class C motorhomes. If there are no letters before the three-digit tire width number, that along with the wheel diameter and Load Range would indicate Truck-Bus or Commercial-type tires as found on Class A RVs.
“235” is the overall width in mm. This works out to about 9-1/4″. The 3-digit metric width number could run from 205 up to 445, depending on the type and expected usage, but the meaning of width is the same.
“75” is the “Aspect Ratio” or the relationship between tire width and the distance from the wheel to the tread as a percentage. So, in this example, the tire would be about 75% as tall above the wheel as the tire is wide or .75 x 9.25″ or about 6.9″. The 2-digit Aspect Ratio number could run from 35 to 85.
R is for Radial
“R”—I do hope that everyone knows this stands for Radial, which is the type of construction used on modern tires. It might also be a “D” for Diagonal, which is the construction used in the older-type “Bias” tires.
“15” is the wheel diameter in inches, with most wheels running from 13″ to 20″ for Passenger-type tires. Light Truck-type tires come in 14″ to 24″ diameter, with most being 16″ on current Pickup trucks used to pull RV trailers.
The “108” or “105” is the Load Index, which is defined as “numerical code associated with the maximum load a tire can carry at the speed indicated by its Speed Symbol under specified service conditions” by the Tire & Rim Association. That is the standardizing body for the tire, rim, valve, and allied parts industry for the United States.
Passenger tires are not used in “dual” or side-by-side position, so they will have only one Load Index. But LT and Bus-type can be used in Dual position, so they will have both Single and Dual Load capacity. If they have a Load Index, there will be two numbers.
Maximum load capacity on all tires
The “S” after the Load Index is the Speed Symbol. Not all tires have the “Load Index” number, but all tires do have the maximum load capacity stated in both Pounds and Kilograms. Also, not all tires have a Speed Symbol. But industry literature found in technical data books suggests a max operating speed in RV application of 75 mph (Speed Symbol L), no matter what the Speed Symbol says.
The “XL” stands for “Extra Load.” No Load Range implies the tie is “Standard Load”—for Passenger tires. For ST, LT, and the Bus-type, the “Load Range” letters start with “C”, “D” and go on up to “H” or “J”. These letters replaced the old “Ply Rating” when tire construction changed from Bias to Radial.
Finally, the “WW or “OWL” stands for Wide White and Outline White Letter. Those are just the fancy white decoration seen on some Passenger tires.
Wheels can be more complicated
Wheels get a bit more complicated as some can be 14″, 15″ or 16″. Then there is a different group that ends in .5, as 17.5, 19.5, and 22.5. These .5″ wheels would be considered HD Truck/Bus type and would all be found on larger and heavier trailers and motorhomes.
So, when shopping for tires you should start off by knowing the “Complete” Size Designation which includes all the numbers and letters from your Original Equipment tires such as “LT235/75R16 114 / 117 L LR-E “. Most, if not all, of this information can be found on your RV Certification Placard or “Sticker”.
If you are going to change any of the above numbers or letters on your tires when buying replacements, be sure the new tires are rated equal or higher for all the Load, Inflation, and Speed symbols. You should never go to a lower load capacity with new tires. In the original example, the new tires are XL and 108, which means higher inflation and higher load capacity.