What all those numbers on your tyre actually mean

Where to find it

All of it is moulded into the sidewall, usually about two thirds of the way out from the wheel. It looks like a jumble; it is five separate pieces of information with one job each.

Worth knowing before you ring anyone: the numbers on the tyre currently fitted are not necessarily the right ones. The authority is the plate in the door shut or the fuel flap.

205 — the width

Millimetres across the tread. A wider tyre puts more rubber down, which helps grip, and also makes the steering heavier and the fuel figures slightly worse.

You cannot change this freely. The wheel and the arch limit it, and the speedometer reads from it.

55 — the profile

A percentage, not a measurement. The sidewall is 55% of the 205mm width, so about 113mm tall.

A lower profile looks better to most people and rides worse to all of them. It also makes pothole damage far more likely.

A 40-profile tyre will find a pothole eventually, and there is very little sidewall between the rim and the tarmac.

91 — the load index

A code, not a weight. 91 means 615kg per tyre, so roughly 2,460kg across the vehicle.

This is where vans get into trouble. A car tyre in the right size can still be below the load rating a van needs, and fitting one is an MOT failure as well as genuinely unsafe loaded.

We will not fit below the rating on the plate, even if you ask. It is the one thing on this list that is not a preference.

V — the speed rating

V is 149mph. Nobody is doing that, which is why people assume it does not matter.

It matters because the rating reflects how the tyre is built and how it handles sustained heat. A tyre rated well below the vehicle's design speed runs hotter on a long motorway trip even at legal speeds.

Which of these you can actually change

The rim is fixed. Width and profile are fixed in practice unless you are changing wheels. Load and speed have a floor set by the manufacturer and you can go above it but not below.

Which leaves brand and tier as the real decision, and that is the one worth spending thought on.

The code nobody mentions

There is a four-digit number on the sidewall giving the week and year the tyre was made. 3422 means the thirty-fourth week of 2022.

Rubber hardens with age whether the tyre is used or not, so a tyre with full tread can still be past its useful life. Most manufacturers suggest replacing at around ten years regardless of wear, and inspecting annually after about five.

It matters most on caravans, trailers and second cars — anything that sits still for long periods and wears its tyres out by the calendar rather than by the mile.

A tyre with 7mm of tread and a 2011 date code is not a good tyre. It is an old one that has not been used.

Mixing sizes and brands

Across an axle, never. The two tyres on the same axle should match in size, tread pattern and ideally brand, because a grip difference side to side is what upsets a vehicle under braking.

Front to rear is more forgiving, and plenty of vehicles leave the factory with different sizes at each end by design. What you should not do is put the newer pair on the front.

New tyres go on the rear, whichever wheels are driven. It sounds counterintuitive and it is what every manufacturer recommends, because losing grip at the back is much harder to correct than losing it at the front.