How to read tyre size: every marking on the sidewall

To read a tyre size, take the main line on the sidewall, such as 205/55 R16 91V, and read it left to right. 205 is the tyre’s width in millimetres. 55 is the sidewall height as a percentage of that width. R means radial construction. 16 is the diameter of the wheel rim it fits, in inches. The last group, 91V, is the service description: 91 is the load index, the most weight each tyre may carry (615 kg), and V is the speed rating (240 km/h). Every other mark on the sidewall, from the date code to the ISI mark, answers a different question, and they are all covered below.

Width and aspect ratio

The first number is the nominal section width: the distance from the outer face of one sidewall to the other, in millimetres, when the tyre is fitted and inflated. A 205 tyre is nominally 205 mm wide. Common Indian car sizes run from 145 on the smallest hatchbacks to 235 and beyond on large SUVs.

After the slash comes the aspect ratio, sometimes called the profile. It is the height of the sidewall as a percentage of the width, not a measurement in its own right. On a 205/55 tyre the sidewall is 55% of 205 mm, about 113 mm tall. A lower number means a shorter sidewall; a 205/45 tyre has a sidewall only about 92 mm tall.

Construction letter and rim diameter

The letter that follows is the construction. R means radial, the way modern car tyres are built. Michelin, which introduced the design, describes it as combining rubber with metal and textile reinforcement to make a strong structure under the tread and flexible sidewalls.

On tyres rated for very high speeds you may see ZR in place of R. The standards allow ZR on radial tyres rated above 240 km/h (speed ratings W and Y), and require it, together with a service description in brackets such as (95Y), on tyres capable of more than 300 km/h.

The number after the letter is the rim diameter in inches: 16 means the tyre fits a 16-inch wheel. You cannot fit a tyre to a rim of a different diameter.

Working out the real size of the tyre

The size code gives you everything needed to work out the tyre’s overall diameter. For 205/55 R16:

• Rim: 16 inches × 25.4 = 406.4 mm
• Sidewall: 205 × 0.55 = 112.75 mm, and there are two of them, top and bottom
• Overall diameter: 406.4 + 225.5 = about 632 mm
• Circumference: about 1,985 mm, or roughly 504 revolutions per kilometre

This matters when you consider a different size. Our tyre size calculator does the arithmetic for any size and compares an alternative with your original.

Load index and speed rating

The service description at the end of the size is two codes run together.

The load index is a number that stands for the most weight a single tyre can carry at the pressure its standard specifies. 91 means 615 kg; 94 means 670 kg; 100 means 800 kg. The figure is per tyre. Our guide to the tyre load index has the full chart for car tyres.

The speed rating is a letter for the highest speed the tyre is rated to sustain while carrying its load: T is 190 km/h, H is 210 km/h, V is 240 km/h and so on. See our guide to tyre speed ratings for the full list of letters.

When replacing tyres, match or exceed both figures from your car’s placard or the original tyres.

XL, reinforced and other letters near the size

A few extra marks sit close to the size:

• XL, Extra Load or Reinforced: a tyre built to carry more load than the standard version of the same size, at a higher inflation pressure. Michelin advises replacing an XL tyre with another XL.
• HL: Michelin’s mark for its high load capacity tyres, designed for heavy cars such as large EVs. Replace HL with HL.
• Tubeless or Tube Type: whether the tyre needs an inner tube.
• M+S and the three-peak mountain snowflake: mud-and-snow and winter markings. Michelin notes that M+S is the maker’s own claim, while the snowflake symbol means the tyre has passed an objective winter-performance test.
• Original equipment marks, such as “AO” for Audi: the tyre version a car maker approved for its model. Michelin recommends replacing an OE-marked tyre with the same marked version.

Maximum load and maximum pressure

Somewhere on the sidewall you will find MAX LOAD and MAX PRESS. Michelin stresses that these are the tyre’s limits, not the right values for your car. The pressure to use is on the placard on the driver’s door pillar or fuel flap and in the owner’s manual.

The date code

Indian rules require every tyre maker to emboss the week and year, or the month and year, of manufacture on each tyre, along with its maximum load capacity (rule 95 of the Central Motor Vehicles Rules).

Most car tyres show the date as the last four digits of the long code that starts with DOT. Under the US rule that defines that code, the first two digits are the week and the last two are the year, so 2319 means the 23rd week of 2019. The code is moulded on one side only, which may be the inner side once the tyre is fitted. Our tyre age checker reads the code for you.

The ISI mark

Since 13 May 2011, a quality control order of the Bureau of Indian Standards has required pneumatic tyres sold in India to carry the ISI mark, with some exemptions notified by the government. The mark shows the tyre has been certified to the Indian Standard that applies to it, which for passenger car tyres is IS 15633.

Tread wear indicators

Finally, look near the shoulder of the tyre for a small symbol. Each maker uses its own, and on Michelin tyres it is a small Michelin Man. It points to the tread wear indicators, raised bars 1.6 mm high in the bottom of the main grooves. When the tread wears down level with them, the tyre has reached the legal minimum. Our guide to tyre tread depth explains how to check.