Radial Tyres

What do the numbers mean?

Radials have a frankly daft system of measurement that mixes metric, imperial and percentages for good measure. Let’s break down 165/70R10 to try and make sense of it all. 

The ‘165’ is the width of the tyre in millimetres. The ‘70’ is the sidewall height (or profile) as a percentage of the tyre’s width. The profile is always quoted as a percentage of the width, this figure is not a measurement in millimetres (or inches for that matter). The ‘R’ stands for radial and the ‘10’ is the wheel diameter in inches.

All radials with a profile lower than 80 will show that profile on the sidewall. For example: 145/70R12, 205/60R13 and 275/55R15. Some, but by no means all, 80-profile radials will show the profile on the sidewall, so 145/80R14 for example. But you could equally see the exact-same size written as 145R14.

Other markings you will see on the sidewall of a 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.

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.