How to read tire markings: uncover the mystery of tire markings

Advance Auto PartsLooking to read the life story of your tires? Well, to a degree, you can. 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. But, because the code is constantly changing to provide more information, 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.

To start, the majority of tires are given a measurement from the ISO metric sizing system.

Discover what an ISO metric tire marking can tell you:

Starting with the ISO code, you will often find a letter(s) on the tire that tells you if the tires are intended for a:

  • P: standard passenger car
  • LT: a light truck
  • ST: special trailer

If you see the letter “T,” it stands for “temporary” and is often written on spare tires and other emergency tire types.

You will also sometimes see a 3-digit number, which provides the tire’s nominal section width. This is usually measured and marked in millimeters, from the widest point of both outer edges.

The aspect ratio is also listed on the tire’s sidewall, and this is 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.

Finally, there will sometimes be a letter on the tire that tells you what the fabric of the tire is constructed of:

  • B is for bias belt, which is great for a rough ride.
  • D is for diagonal.
  • R is for radial. Radial is one of the most prolific tire materials on the market.
  • If there is no letter marking, then you likely have a cross ply tire.

The load index is a tire marking that 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.

Finally, once you know the load index number of your tire, you can begin to pay attention to its speed rating. This tells you how fast you can go based upon your specified load index. So:

  • 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.

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 what your car can do.

Editor’s note: Shop Advance Auto Parts for a wide variety of tire gauges, tire repair tools, accessories and more. Buy online, pick up in store. 


Graphic courtesy of Consumer Reports.

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