How to Read Numbers on Car Tires

The numbers and letters on the sidewall of car tires can be difficult to read and understand. Once they are understood, the numbers can help determine the tire type, tire width, aspect ratio, construction type, wheel diameter, load index, speed rating and the maximum inflation level of the tire. This guide starts at the tire type letters and moves clockwise around the tire.

Instructions

1. Find the tire type by locating the letters "P" or "LT" on the sidewall of the tire. The "P" designates that the tire is for a passenger car. The "LT" means the tire is designed for a light truck.

2. Determine the tire width by reading the numbers directly following the tire type letter. This number is likely to be 3 digits and followed immediately by a "/."

3. Identify the aspect ratio of the tire from the numbers after the "/." This is the height by the width of the cross-section. This number is 2 digits and tells the percentage of the height to the width. For example, 55 would mean the height is equivalent to 55 percent of the width of the tire.

4. Find the type of construction of the tire by the letter following the aspect ratio. The letter can either be an "R" or a "B." The letter "R" stands for radial and means that the layers of fabric that make up the tire run lengthwise across the tire. The "B" stands for bias and indicates that the layers run diagonally.

5. Identify the wheel diameter by reading the number immediately following the construction letter. This 2-digit number tells the width of the tire from one end to the opposite end.

6. Move clockwise around the tire, and locate the next number. This number reveals the load index or the number of pounds the tire can hold when fully inflated.

7. Read the letter immediately following the load index to determine the speed rating. There are 13 different speed ratings. These ratings are M, N, P, Q, R, S, T, U, H, V, W and Y.
   The M rating is the lowest with a maximum speed of 81 miles per hour (MPH). The speeds go progressively higher, rising about 6 or 7 MPH at a time until V. The V maximum speed is 149 MPH, followed by W at 168 MPH and, finally, Y at 186 MPH.

8. Determine maximum inflation level by reading the number followed by the pounds per square inch (PSI). Use a tire gauge to measure the current PSI of the tire before filling and never fill past that amount.

Tips & Warnings

• Never exceed the speed limit when driving based upon your speed rating. This rating only designates the maximum speed at which the tire can safely be driven before permanent damage is done. It is not a recommended speed limit.