How To Read Car Tires

Your car’s tires will come with a combination of numbers and letters, with each one indicating a certain element. By studying these numbers, you can learn about the tire’s width, rim diameter, and even details about the internal construction of the tire.  

First, you’ll want to locate these letters and numbers, which can be found on the tire itself. However, they can also be found in the owner’s manual, driver’s door jamb, glove box, or gas tank hatch.

The First Letters

You’ll first observe the letter “P” or “LT” at the beginning. The “P” will be found on passenger vehicles, while the “LT” will be on light trucks. These simply indicate “P-metric” or “LT-metric” tires and serve as a way to show what type of tire it is.

Your trailer tires and spare tires will have different letters. The spare will have a “T” for “temporary,” and trailer tires will have an “ST” for “special trailer.”

Three Numbers Before the Slash

Once you identify the type of tire you have, you’ll then see a three-digit number. This is the millimeter measurement for your tire’s width that has been measured across the sidewalls.  

Number After the Slash

This number symbolizes the aspect ratio of the tire. To determine the aspect ratio, the tire’s sidewall height is then divided by its width. The resulting number is the percentage of tire height to its width. For example, if the tire has an aspect ratio of 75, the tire’s height is 75% of its width.

Construction Letter

After the aspect ratio, you will then see another letter. This will either be an “R” or a “D.” The “R” is for radial tires and is much more common than its counterpart. This means that the internal construction of the tires, which consists of cords known as plies, have been placed in a radial direction. This construction provides more stability on the road.

You may find the letter “D” on trailer tires. That letter stands for diagonal and means that the plies have been placed diagonally to each other.

Rim Diameter

Once you know your tire’s construction, you’ll then find out its rim diameter, which is measured in inches. Measured horizontally across the tire, it often ranges anywhere from 14 inches up to 22 inches.

Load Index

The last number on the tire is the load index and can be either two or three numbers. This number corresponds to a specific chart that will tell you how much weight your tires can handle.

Speed Rating

The speed rating is the last letter you will see. It also corresponds to a specific chart that will let you know just how fast your tires can travel.