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How to Read Tire Sizes

How to Read Tire Sizes

Have you ever observed the tires of your vehicle carefully? If you have, then surely you must have noticed a series of numbers and letters inscribed on the rubber. Do you know the meaning of this series, and its significance? If you don't, then go through this WheelZine article to find out.
Saurabha Palekar
Last Updated: Feb 28, 2018
Many people don't know the meaning of the numbers and letters mentioned on the sidewalls of their vehicle's tires. In fact, most don't even know that they exist! However, these are quite important, as they provide useful information regarding, among other things, the size, air pressure requirement, load capacity, and construction type of each individual tire.

Information about the tire size can be found on the sidewall of a tire, typically inscribed between its name, and the name of its manufacturer. It is an alphanumeric series, which differs from tire to tire based on its class and other specifications, such as the type of construction, aspect ratio, etc. In the following lines, we will explain the meaning of each element in this series in order to help you read and comprehend the information pertaining to the size of the tire on your vehicle.

Tire Size: Explanation
  • The series normally begins with the class of the tire, usually the letter 'P', which denotes that it is a 'Passenger Car Tire'. It can also be 'LT', which indicates 'Light Truck', 'T' for 'Temporary Spare', or 'ST' for 'Special Trailer Service'.
  • The three digit number followed by the letter 'P' denotes the width of the tire in millimeters.
  • The next number following the width represents the 'Aspect Ratio' of the tire, which is the ratio of the height of the tire to its width. For a regular tire, this ratio lies between 50 to 70.
  • After this number, you will find a letter which gives the type of construction of the tire. This letter is usually 'R', which stands for 'Radial Construction'. It can also be 'D', which stands for 'Bias Ply Tires' or 'Diagonal Tires', or 'B', which indicates 'Belted Bias Tires'.
  • The number which comes after 'R' denotes the 'Diameter of the Wheel' in inches. Usually, the diameter of the wheel ranges from 13 to 22.
After a gap, there is another number followed by a letter, which are together called the service description. It is a combination of load index and speed symbol.
  • The number in this combination stands for 'load index'. It indicates the maximum weight a tire can carry under certain service conditions, and ranges from 71 (761 lb) to 110 (2,337 lb).
  • The letter which follows this number is the 'speed symbol' or 'speed rating'. It is the maximum rated speed for the tire at the specified load.
The recommended air pressure is written on the tire rim.

Tire Size Example

If the number on the tire is 'P205/75R15 89H', then the meaning of its tire size can be decoded as following:

A passenger car tire of width 205 millimeters, having an aspect ratio of 75, and radial construction which has 15 inches of wheel diameter, and can carry 1,279 pounds of load at the maximum speed of 130 MPH (210 KPH), under certain service conditions.

Knowing how to read the tire number is especially important when you change the tire. It is not safe to use a tire which does not meet your vehicle's tire specifications. Using a tire of inappropriate size can also affect the speedometer accuracy, power, and traction control of your vehicle. Now that you know how to read tire sizes, it will not only come in handy when you are changing the tires, but also prove useful in selecting an appropriate one.
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