How-To25 January 2026·4 min read

How to Read the Numbers on Your Tyre Sidewall — Decoded

The string of numbers and letters on your tyre sidewall looks complicated, but each element tells you something important. Here is how to decode them in under five minutes.

A typical tyre sidewall marking might read "205/55 R16 91V". Each component carries a specific meaning. The first number — 205 — is the tyre width in millimetres measured across the tread. The second number — 55 — is the aspect ratio, meaning the sidewall height is 55% of the tyre width. The letter R indicates radial construction (the standard for virtually all modern car tyres). The number 16 is the wheel rim diameter in inches.

The numbers that follow the rim diameter relate to the tyre's load and speed ratings. In this example, 91 is the load index, which refers to a maximum load capacity per tyre (91 equates to 615kg). The letter V is the speed rating, indicating the maximum sustained speed the tyre is rated for (V means up to 149mph). Using a tyre with a lower load index or speed rating than specified for your vehicle is not recommended and may affect insurance validity.

Additional markings you may encounter include XL or RF, indicating reinforced or extra-load construction for heavier vehicles. M+S (mud and snow) indicates some winter capability. The four-digit DOT production code tells you when the tyre was manufactured. When ordering replacement tyres, always use the exact size specification from your vehicle handbook or existing tyre — Castle Mobile Tyres will cross-reference your registration to confirm the correct size if needed.