RV Tire Safety: What are the letters and numbers on the tire sidewall?

Many times I am asked what seems like a simple question, such as “How much air should I run in my 16″ tire?” Or “What load can my trailer 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?” Complete tire size information is needed to get a correct answer on load and inflation.

While I want to answer your questions, 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 we need. To help us do a better job of answering your questions, you can help by giving us the needed details.

Here are some examples of how you can help and get the answer you want faster by providing the complete size information from the tire sidewall when you ask a question.

A P235/75R15 105S is rated at 2,028 lbs. at 35 psi maximum. This would be a “Standard Load” passenger tire. A P235/75R15 108S XL, or Extra Load, is rated at 2,183 lbs. at 41 psi max.

Breakdown of size information from tire sidewall

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 the Load Index and the Speed rating makes up the Service Description.

Special note on Passenger-type tires when applied to multipurpose vehicles such as trailers, pickups, and SUV: The load rating is reduced by dividing the stated load by 1.10.

Special Trailer tires

Moving on to ST-type or Special Trailer tires. All the rest of the numbers and letters mean the same as for “P” type but the load and inflation tables are different. LT-type tire is another “type” of tire intended for use on Light Trucks aka pickups (generally 10,000# and lower GVWR). Finally, we get to “truck type” tires that have no preceding letter. These are on 19.5″ and 22.5″ wheels. Again, their load and inflation tables are different than P, ST and LT type tires.

The final letter is used for “Load Range”. These start with “XL” on passenger-type tires but all heavier tires will have “C” or “D” and on up. You can think of the Load Range letters as replacing the Ply Rating numbers of the 1950s – 1970s bin Bias or Diagonal construction tires.

All the letters and numbers make up the complete tire size information needed to get a correct answer on load and inflation.

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.

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 his blog at RVtiresafety.net or on RVtravel.com.

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