Tuesday, October 3, 2023


What do all those letters mean in the tire size?

A fairly consistent problem I run into when I try to provide answers about specific tire applications is the fact that I am usually only given part of the tire information I need to provide an informed answer. I have been asked, “What load can my 16” tires carry?” or “I have 8-ply tires. How much load can they carry?” or “Can I replace my 235/70R16 tires and carry more load with a 245 tire?” While I really want to help you get the information and answer you seek, I do not want to guess what tire you have as I may guess wrong and give you incorrect information.

The reality is that the numbers alone are not all the information needed. So to help me do a better job of answering your questions, you can provide the needed details accurately. Here are some examples of how you can help us all if you provide the complete size information when you ask a question.

Understanding tire size information

A P235/75R15 105S is rated at 2,028 lbs. at 35 psi maximum. This would be a “Standard Load” tire. A P235/75R15 108S XL, or Extra Load, is rated at 2,183 lbs. at 41 psi max. Just to confuse everyone, there are a number of “Euro-Metric” sizes showing up on vehicles such as “235/75R15” that could be Standard Load or Reinforced, with the “reinforced” being like US “Extra Load” passenger tires, or more likely “235/75R15C”. Note there would not be a “LR-C” or “Load Range C” for the European tires, as the “C suffix stands for “Commercial,” which we could consider more like a U.S. “Light Truck” tire.

We can break down the different parts of the complete size designation as follows: The P stands for Passenger. The 235 is the width of the tire in millimeters and the 75 is the Aspect Ratio of how tall a tire is relative to how wide. In practical terms, it is how close the wheel is to the road. The “R” of course stands for Radial. If the tire were Bias, as tires were before the introduction of Radials, it would have a D for Diagonal. Don’t ask me why they didn’t choose the letter B. I have no idea.

We all know the 15 is the rim diameter. The 105 or 108 is the Load Index. You can consider any passenger tire to be Standard Load unless marked “XL” or “Extra Load”. The “S” in the size above is the speed or handling rating. Passenger tires are not rated for dual application. The combination of Load Index and the Speed rating make up the Service Description.

Asking questions about tires

When you ask a question about a tire, don’t worry about what all the letters and numbers mean. Just look at your tire and copy all of the information and leave it up to me and other tire engineers to decipher the mumbo-jumbo. If you tell us your tire size is a P285/35ZR19 87Y we will know this ultra wide, very low aspect ratio, large rim tire with a high speed rating indicates you are driving a Corvette, and that those tires are not meant to be used on your RV.

Yes, this is complex, but you DO NOT have to remember or understand all this nomenclature. What may be easiest is to use your phone and capture a few images of the information on the sidewall of your tires and include them in an RV forum post or with your question on tires.

Roger Marble

Check out my Blog www.RVTireSafety.Net

Have a tire question? Ask Roger on his 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 RVtravel.com.


Roger Marble
Roger Marblehttp://www.RVTireSafety.net
Retired Tire Design and Forensic Engineer w/50+ years of experience. Currently has Class-C RV. Previous Truck Camper, Winny Brave, Class-C & 23'TT. Also towed race car w/ 23' open trailer and in 26' Closed trailer. While racing he set lap records at 6 different tracks racing from Lime Rock CT to Riverside CA and Daytona to Mosport Canada. Gives RV and Genealogy Seminars for FMCA across the USA. Taught vehicle handling to local Police Depts


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2 months ago

Another thing is the ST marking, (special trailer). They have heavier cords than standard tires to accommodate the extra weight being carried and do not flex as much as a standard tire. That, along with the weight rating determines the max weight the trailer tries are capable of carrying. But not all trailer tires are the same even though the ratings are the same. I replaced my Trailer King tires with Goodyear Endurance. The sidewalls are much thicker and stiffer. Both sets were Load Range E. My tire shop was surprised just how stiff they were compared to the Trailer Kings.

Bob P
2 months ago

I would also add for clarification, the 235 millimeters is the cross section width not the tread width, the widest part of the tire. Enjoy your articles!

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