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Tire Hieroglyphics: Understanding the Numbers on Your Trailer Tires' Side Walls

May. 8 2025 Guides By JTI

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Tire Hieroglyphics - Understanding Markings on Your Trailer Tires Side WallEver wondered what all those numbers and patterns mean on your trailer tires and wheels?  Part numbers like “ST205/75R15 101/97M” might seem like hieroglyphics at first glance, but they reveal important details about your tire’s size, load capacity, and fit.  Understanding these numbers and patterns is key to ensuring your trailer performs safely and efficiently. 

Let’s break it down: 

Size, Shape and Construction

Understanding Tire Codes - Tire Side Wall Numbers and Letters Infographic by JTIIf you embed this infographic on your site, please link back to https://www.jtiinc.com!


Let's analyze the tire code from the photo: ST205/75R15  101/97M

  • ST: Tire type or vehicle class, indicates type of vehicle the tire should be used for. ST is for Special Trailer.
  • 205/75: The first part of a tire code indicates the tire’s dimensions.  The number 225 represents the section width, the width of the tire in millimeters, while 75 refers to the aspect ratio, which is the height of the tire’s side wall as a percentage of its width.
  • R: Radial (Tire’s Construction). Could also be B for Bias-belted, or D or – for bias-ply laid diagonally.
  • 15: Rim Diameter. This number tells you the tire is designed to fit a 15-inch rim diameter.
  • 101/97: This is the load index. Normally one number that you look up on an index to see the maximum load weight on that tire. 101 translates to 1819 lbs. See the load index table below.

    When you have two numbers with a slash like 101/97M, the tire is designed for a light truck or trailer. The first number is the load rating when the tire is used alone, and the second is when it’s used as a dual tire. Why is the second one lower? Most often the dual rating is lower in case of situations where one pair of tires fails and the other pair must carry the entire load.
  • M: Speed Rating. A tire's speed rating indicates how fast it can go under normal conditions before performance starts to suffer. Speed ratings go from A1 to Y, with A1 having the lowest rating and Y being the highest, and there are some exceptions/anomalies (e.g. H being in the wrong place). If you're looking for a speed rating table, we've got it at the bottom of this article!

You may see something like “6-Lug 10 Ply” written on the side of the tires as well. Here are some notes on that. 

 Lug Patterns

  • 5-lug patterns can vary in spacing, which is why you’ll see options like 5 on 4.5, 5 on 4.75, or 5 on 5.5.  Matching your lug pattern is crucial for ensuring a safe and secure connection between your wheel and trailer.
  • 6-lug and 8-lug patterns are standardized and commonly used for heavy trailers, offering more stability and load-bearing capacity.

Tire Ply 

  • Tire ply is another critical factor when choosing the right tires for your trailer.  The ply rating tells you how strong your tire is and how much weight it can safely carry.
  • 8-ply tires offer solid durability and are often used for lighter loads.
  • 10-ply tires, such as the 225/75-15 6-lug 10-ply, are ideal for medium-duty trailers.
  • For heavy-duty trailers, you’ll 14-ply tires like the 235/80-16 8-lug 14-ply, which provide exceptional strength and durability.

Tire Date Code and DOT Code 

Tire Date Code InfographicIf you embed this infographic on your site, please link back to https://www.jtiinc.com!

Here’s a useful bonus. Look for where it says DOT on the sidewall. After DOT there will be a series of up to 13 characters. The last four will be the date code. First two digits are the week of the year that the tire was made. Second two digits are the year.

So in this example, 4224, the tire was made in 2024, in the 42nd week of the year. Note that it’s best practice to replace your tires every five years. With lighter use you can push it to seven, but you might be pushing your luck.

By understanding these tire and wheel specifications, you can ensure your trailer performs safely and efficiently, no matter what you’re hauling.

If you have any questions please stop in and we will be happy to explain anything regarding tires and wheels, and what is your best option. Or give us a call at 440-232-4311! (Also, we can provide trailer tires and wheels - some of our bestsellers are here. )

REFERENCE TABLES BELOW FOR TIRE SPEED RATINGS and LOAD INDEX.

 

Tire Speed Ratings Table

Refer back to this speed ratings table whenever you need to read the speed ratings on your tires. If you embed or repost these tables please link back to JTIInc.com!

Load Index Table

Comprehensive Tire Load Index Table from JTI Trailer & Hitch