How to Read Your Tyre Sidewall: A Complete Guide

Decoding the Numbers on Your Tyre Sidewall

Every tyre carries a series of numbers and letters on its sidewall that tell you its exact specification. Understanding these markings means you can order the correct replacement tyres with confidence. Take a typical marking like 225/45 R17 91W. The first number (225) is the tyre width in millimetres measured across the tread face. The second number (45) is the aspect ratio — the height of the sidewall expressed as a percentage of the width. So in this example, the sidewall is 45% of 225mm, which is approximately 101mm tall.

The letter R indicates radial construction, which is standard on all modern tyres. The number 17 is the rim diameter in inches — this must match your wheel exactly. The number 91 is the load index, which tells you the maximum weight the tyre can carry. A load index of 91 corresponds to 615kg per tyre. Finally, the letter W is the speed rating — W means the tyre is rated to a maximum continuous speed of 270 km/h (168 mph).

Other Important Sidewall Markings

Beyond the size code, you will see the brand name and tyre model, plus an arrow indicating the correct direction of rotation for directional tyres. Some tyres are marked with OE codes from car manufacturers (like MO for Mercedes or AO for Audi), indicating the tyre was approved as original equipment for that brand. If your car came with OE-marked tyres, it is worth replacing like for like where possible.

The DOT code is a legal requirement in all markets and begins with the letters DOT followed by a series of numbers and letters. The last four digits are the most important for consumers: they tell you the week and year the tyre was manufactured. For example, 2423 means the tyre was made in the 24th week of 2023. When Just Mobile Tyres supplies your replacement tyres, you can be confident you are receiving fresh stock — we never supply aged or out-of-date tyres.