Look closely at the side of a car tyre and you will see a line such as 185/65 R15 88H. To a new driver this looks like a serial number, but it is actually a short code that tells you the width of the tyre, the height of its sidewall compared with the width, the type of construction, the rim diameter it fits, how much load it can carry and the maximum speed it is designed for. When you replace a tyre, matching this code with what the vehicle maker recommends is one of the simplest safety steps you can take.
The first number, 185 in the example, is the width of the tyre in millimetres, measured from one sidewall to the other. The number after the slash, 65, is the aspect ratio, also called the profile. It means the height of the sidewall is 65 percent of the width. A lower profile means a shorter sidewall, which is common on sporty cars. A higher profile usually gives a softer ride on rough roads. Changing these numbers without advice can affect speedometer accuracy, handling and clearance.
Construction and rim size
The letter R stands for radial construction, which is the most common type on modern cars. In a radial tyre, the layers of cord run across the tyre at right angles to the direction of travel. The number after R, 15 in the example, is the diameter of the wheel rim in inches that the tyre is made to fit. A tyre marked R15 must go on a 15 inch rim. Fitting a tyre on the wrong rim size is unsafe and usually not even possible.
You may see other letters in some markings, such as P for passenger car in some systems, or LT for light truck. Two wheeler tyres have their own formats, which may use different units or show the rim size in a different way. Read the owner’s manual for your vehicle. It lists the tyre sizes approved by the maker, and sometimes more than one option for different variants.
Load index and speed rating
After the rim size you usually see a number and a letter, such as 88H. The number is the load index. It is a code that stands for the maximum load the tyre can carry at the right pressure, according to standard tables. A higher number means a higher load capacity. The letter is the speed rating, a code for the maximum speed the tyre is designed to sustain under specified conditions. Letters such as S, T, H and V are common on cars, with each letter standing for a defined speed.
When buying a replacement, the load index and speed rating should be at least what the vehicle maker recommends. Choosing a lower load index on a vehicle that often carries a full family and luggage can be risky. The tables for converting codes into kilograms and kilometres per hour are available from tyre makers and standards bodies, so this article does not reproduce them.
Other useful markings
Tyres also carry a manufacturing date code, often a four digit number where the first two digits are the week and the last two digits are the year of manufacture. This helps you avoid buying very old stock. Tyres sold in India are also expected to meet Indian standards, and carry the ISI mark under the Bureau of Indian Standards certification scheme for tyres. Other markings may show whether the tyre is tubeless, the direction of rotation, or the tread wear indicator positions.
Tread wear indicators are small raised bars inside the grooves. When the tread wears down to the level of these bars, the tyre should be replaced, because grip on wet roads falls sharply. Check tyre pressure when the tyres are cold, using the figures printed on the sticker near the driver’s door or in the manual, not the maximum pressure printed on the sidewall. Correct pressure improves safety, tyre life and fuel use.
In short, the tyre size code tells you width, profile, construction, rim size, load index and speed rating. Read it before buying, follow the vehicle maker’s recommended sizes, check the manufacturing date and the ISI mark, and keep the right pressure. When in doubt, ask an authorised dealer to show you the code on your old tyre and the new one side by side.
Source: Bureau of Indian Standards, Ministry of Road Transport and Highways
