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Guide

Tyre and wheel sizes: reading a mixed-unit code

A tyre marked 225/45 R17 uses three different measurement conventions in six characters: millimetres, a percentage and inches. It is one of the last places where imperial and metric sit side by side on the same label, and misreading it leads to buying a tyre that physically will not fit.

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What each part of the code means

The first number is the section width in millimetres — 225 means the tyre is 225 mm across at its widest point. The second is the aspect ratio, a percentage rather than a measurement: 45 means the sidewall height is 45 per cent of the width, so 101 mm here.

The letter R denotes radial construction, which is what virtually every road tyre uses today. The final number is the rim diameter in inches, and this one is not negotiable: a 17-inch tyre fits a 17-inch wheel and nothing else. Seventeen inches is 43.18 cm, but nobody in the trade uses the metric figure.

Overall diameter combines the two systems. Rim diameter of 17 inches is 431.8 mm, plus twice the 101 mm sidewall gives 633.8 mm. That total matters because a change of more than about two per cent affects speedometer accuracy and, on modern cars, the stability control calibration.

Key facts

Width in millimetres

The first number is the section width, so 225 means 225 mm across.

Aspect ratio is a percentage

45 means the sidewall is 45 per cent of the width — 101 mm on a 225 tyre.

Rim size in inches only

A 17-inch tyre fits a 17-inch wheel; 17 inches is 43.18 cm.

Diameter mixes units

Rim in millimetres plus two sidewalls gives the overall diameter.

Two per cent rule

Stay within two per cent of the original diameter to keep the speedometer honest.

Load and speed codes

The trailing characters, such as 91V, cap weight per tyre and maximum speed.

How to get it right

1

Read the sidewall

Note width, aspect ratio, rim diameter and the load and speed codes.

2

Convert the rim

Multiply inches by 25.4 for millimetres — 17 inches is 431.8 mm.

3

Calculate sidewall height

Multiply the width by the aspect ratio as a decimal.

4

Compare diameters

Add the rim diameter to twice the sidewall height and keep the change under two per cent.

Questions people ask

A tyre 225 mm wide, with a sidewall 45 per cent of that width (101 mm), radial construction, fitting a 17-inch rim.

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