Your Tyre Size: A Complete Guide

Tyre size encodes width, aspect ratio, construction, rim diameter, and often load and speed indices. Getting it wrong affects clearance, gearing, speedometer accuracy, and certification.

Breaking down 225/45R17 91W

225 mm section width, 45% sidewall height relative to width, R radial, 17-inch rim. 91 is load index, W is speed rating. Some tyres add XL, RF (run-flat), or M+S markings.

Where to find the right size

The placard lists front/rear options if they differ. The spare may differ — especially space-savers. Always cross-check with what is actually fitted if the car is modified.

Changing size

Plus-sizing (larger rim, lower profile) is common for aesthetics but must preserve similar overall diameter. Use a tyre size calculator or professional fitment guide.