How to Read Tire Markings and Sidewall Codes

Each car tire reveals its uses and specs through a code that consists of numbers and letters, a code that is usually found in the tire sidewall. It may look like a hodgepodge that's as clear as an obscure foreign language. Fortunately, with a little help, anyone can understand what’s written on their tires. And, once you do, it can make all the difference in what you purchase and how you use the tires. Here are the basics of what all those figures mean.

Wheel and tire

Source | Flickr


Reading Tire Markings

ISO metric sizing system

You'll often find a letter(s) on the tire that tells you what the tires are intended for.

  • P is for a standard passenger car.
  • LT is for a light truck.
  • ST is for a special trailer.
  • T is for temporary, usually indicating a spare tire.

Nominal section width

This 3-digit number indicates the widest point of both outer edges in millimeters.

Aspect ratio

Usually a two- or three-digit number that is written as a percentage. If this is not listed on your tire, then your tire’s aspect ratio is the standard 82%, which means that the sidewall height is 82% of its width. Any other measurement will be marked.

Fabric codes

This indicates your tires' design.

  • B is for bias belt, which is rarely found in passenger or light tires in the U.S. today.
  • D is for diagonal.
  • R is for radial, and is one of the most prolific tire designs on the market today.
  • If there is no letter marking, then you likely have a cross ply tire.

Load index

Denotes how much a tire can carry. For example, a code of 60 means a tire can carry up to 550 pounds. The highest code is 125, which can carry approximately 3,600 pounds per tire.

Speed Rating

This tells you how fast you can drive on your tires. Code A1 means that you can go 3 mph at the specified load index. This goes up to code Y, which allows you to travel up to 186 mph. Of course, you still have to drive within legal posted speed limits.

These are the basics of how to read tire markings. They are important to know when you’re shopping for replacement tires or just need to be educated on your car's capabilities. 

Are your tire codes' clear as mud? Leave us a comment.

Pro Tip

If you up-size or down-size your tires from OEM specs, you may need to have your speedometer adjusted accordingly. If in doubt, you should always consult with a tire specialist at a reputable tire outlet regarding recommended tire sizing and options.

Last updated June 6, 2018