Tyre Markings Decoded: Reading the Numbers on Your Sidewall

The Anatomy of a Tyre Code

That string of characters moulded into your tyre's sidewall — something like P215/65R17 98H — follows a standardized format used across the industry. Each segment communicates a specific physical characteristic. Reading it correctly means you'll always know whether a replacement tyre is truly compatible with your vehicle.

The code typically begins with a type prefix. "P" stands for passenger vehicle; "LT" indicates light truck. Some tyres carry no prefix, which generally signals a European metric sizing standard. This distinction matters because LT-rated tyres have higher load requirements than P-rated ones.

Breaking Down Each Number and Letter

After the type prefix, the numbers follow in a predictable sequence:

1. Section width — The first three digits (e.g., 215) give the tyre's width in millimeters, measured from sidewall to sidewall when properly mounted.
2. Aspect ratio — The two digits after the slash (e.g., 65) express the sidewall height as a percentage of the section width. A 65 aspect ratio means the sidewall height is 65% of 215 mm, or about 140 mm.
3. Construction type — The letter "R" means radial construction, which is by far the most common type on modern passenger vehicles.
4. Rim diameter — The following number (e.g., 17) is the wheel diameter in inches. This must match your wheel exactly.
5. Load index — The number after the space (e.g., 98) is a coded value. A load index of 98 corresponds to a maximum load capacity of 1,653 lbs per tyre — always reference a load index chart for the exact figure.
6. Speed rating — The final letter (e.g., H) indicates the maximum sustained speed the tyre is engineered for. "H" corresponds to 130 mph. This is a design parameter, not an invitation to exceed legal speed limits.

The DOT Code and Manufacturing Date

Elsewhere on the sidewall, you'll find a string beginning with "DOT" — the U.S. Department of Transportation compliance identifier. The last four digits of this sequence are the most actionable piece of information for most drivers.

Those four digits represent the week and year of manufacture. For example, "2319" means the 23rd week of 2019. Tyres degrade from the inside out as rubber compounds oxidize over time, even when tread depth looks acceptable. Most vehicle and tyre manufacturers recommend replacing tyres that are 6 to 10 years old regardless of visual condition.

If you're purchasing used tyres or a vehicle with unknown service history, locating the DOT date is one of the first checks worth making. See our tyre care guidance for more on how age affects tyre integrity.

Putting It All Together

Once you can read a tyre sidewall fluently, you're in a much stronger position during any tyre-related conversation — whether at a service centre or when verifying that a replacement is correct before it's mounted. Cross-reference the size against your vehicle's door jamb placard, confirm the load index meets or exceeds the original specification, and check the DOT date if age is a concern.

For broader vehicle literacy, our dashboard warning lights reference covers the symbols that often appear alongside tyre-pressure alerts — useful reading for any driver building their automotive knowledge base. And if you enjoy decoding vehicle identifiers, reading a VIN follows a similar logic of structured, position-based information.

Tyre markings exist to protect you — they encode the engineering decisions made to keep a specific tyre safe within defined limits. Understanding them is one of the more practical steps any driver can take toward informed vehicle ownership.