The tyre size is moulded into the sidewall. You may see several codes, but the main size normally follows a pattern similar to 225/45 R18 95Y. Each part describes a different requirement.
225 — tyre width
The first number is the nominal section width in millimetres. In this example, the tyre is approximately 225 mm wide at its widest section when fitted to an approved wheel. It is not simply the tread width.
45 — aspect ratio or profile
The second number is the sidewall height expressed as a percentage of the width. Here, the sidewall height is 45% of 225 mm. A smaller number usually creates a shorter sidewall, but the complete tyre and wheel specification must still match the vehicle requirements.
R — radial construction
The letter R means radial construction, which is standard for most modern passenger-vehicle tyres. This letter does not mean the wheel radius.
18 — wheel diameter
This number is the rim diameter in inches. An 18-inch tyre must be fitted to a compatible 18-inch wheel. Width and profile alone are not enough to confirm a replacement.
95Y — load index and speed rating
The number 95 is a load index, not a weight in kilograms. It refers to a standard table that specifies the maximum load per tyre under defined conditions. The letter Y is a speed rating. A replacement should meet the vehicle manufacturer’s required load and speed specification; choosing a lower rating can be unsafe or unsuitable.
Other markings you may see
XL or Extra Load
This indicates a tyre designed for a higher load capacity at specified pressures compared with a standard version of the same size.
Run-flat markings
Manufacturers use different abbreviations for run-flat technology. If your vehicle currently uses run-flat tyres, mention this before choosing a replacement.
Manufacturing date code
A four-digit code within the identification marking represents the production week and year. For example, 2425 would mean week 24 of 2025. The date code is only one part of tyre-condition assessment; storage, heat exposure, damage and wear also matter.
Vehicle-specific approvals
Some tyres carry approval markings for particular vehicle manufacturers. Matching the original specification can be important for performance, handling and warranty requirements.
Should both tyres on an axle match?
For predictable handling, tyres on the same axle should generally match in size, construction, load and speed rating, with compatible tread characteristics and similar condition. Four-wheel-drive and all-wheel-drive vehicles may have stricter requirements for tyre circumference and tread-depth differences. Check the vehicle handbook or manufacturer guidance.
Where else can you confirm the correct size?
The current sidewall is useful, but it may not always represent the original specification. Check the driver-door label, fuel-filler area, vehicle handbook or manufacturer information. Some vehicles list several approved sizes depending on wheel package.