How to Read Tire Sizes

It is certainly wrong to assume that everyone knows the meaning of the numbers written on the tire. In fact, many of us do not even know that they exist! The numbers written on the sidewalls of the tire gives us useful information about the size, air pressure, load capacity and construction type of the tire. Just follow these simple tips and you will definitely understand the meaning of the numbers. The tire size is written between the name of the tire and the name of the tire-manufacturer. It differs according to the class and other specifications like radial construction and the aspect ratio of the tire.

Tire Size Explanation

• Normally it starts with the class of the tire, usually letter 'P' which denotes that it is a 'Passenger Car Tire'. It can also be 'LT' which indicates 'Light Truck' or 'T' for 'Temporary Spare' or 'ST' for 'Special Trailer Service'.
• The number followed by the letter 'P' denotes the width of the tire in millimeters.
• The second number followed by the first one denotes 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 is usually 'R', and it 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. It is called the service description. It is a combination of load index and speed symbol.

• The number stands for 'Load Index', which indicates the maximum weight a tire can carry under certain service conditions. It ranges from 71 (761 Pounds) to 110 (2337 Pounds).
• 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, usually by the tire name. Now if the number on the tire is "P205/75R15 89H", then its tire size meaning will be as follows: A passenger car tire of width 205 millimeters having an aspect ratio of 75 and radial construction, has 15 inches of wheel diameter and can carry 1279 pounds of load, at the maximum speed of 210 km/h, under certain service conditions.

Reading the tire number is important when you change your tire. It is not safe to use a tire which does not meet your vehicle tire specifications. Using a tire of inappropriate size can also affect the speedometer accuracy, power and traction control. Knowing your vehicle's tire size is not only useful when you are changing the tire, but also in selecting an appropriate one.