Find the main size code
Look at the writing moulded into the tyre sidewall. The main size usually follows a pattern such as 225/45 R17 94W. Sending the complete code helps a fitter identify a compatible replacement and avoids delays caused by bringing the wrong tyre.
What each part means
In 225/45 R17, 225 is the nominal tyre width in millimetres. The number 45 is the aspect ratio: the sidewall height is 45 per cent of the tyre width. R describes radial construction and 17 is the wheel diameter in inches. The following number and letter, such as 94W, identify the load index and speed rating.
Extra markings can matter too. XL generally identifies an extra-load tyre, while run-flat tyres use manufacturer-specific markings. If you are unsure, send a clear photograph of the full sidewall rather than guessing.
Check more than one place
Do not rely only on an old invoice or an online registration lookup. Wheel sizes can be changed and some vehicles use different sizes at the front and rear. Reading the affected tyre is the quickest starting point, while the vehicle handbook or door-pillar label can help confirm the approved specification.
This guide is general information, not a remote tyre inspection. If the vehicle or location is unsafe, stop driving and seek appropriate professional assistance.