Radial Metric Tyres

What do the numbers mean?

Metric radial tyres are measured in millimetres for both the section width and the rim width. If we take a 190/55R340, this has a 190mm section width. The profile is 55, meaning the sidewall height is 55 per cent of the width. This figure is not a measurement in millimetres. The ‘R’ stands for radial and the ‘340’ denotes the rim diameter in mm.

Other markings you will see on the sidewall of a metric radial include a load rating quoted as a two- or three-digit number and a speed rating illustrated by a letter. These charts will help you decipher the markings.

Cor! These are expensive, can’t I just fit a regular tyre that’s nearly the right size?

No! Definitely not. You can only use metric tyres on metric wheels. If you want to use ‘regular’ tyres you will need to change the wheels.

What is a radial anyway?

The cords in the fabric used in the plies of a radial tyre are positioned around 90 degrees to the direction of tyre travel. By varying the angle of the cords, the tyre carcass can be stiffened for high-performance applications to improve steering response.

Can I use radial and crossply tyres on the same car?

Arcane legislation allows (incredibly) the use of radials at the rear and crossplies at the front. Never the other way round and never on the same axle. 

We’ll add another ‘never’ to that list: Never mix radials and crossplies on any vehicle regardless of what the law says. The tyres have very different handling characteristics and you are asking for trouble if you do it.