What do the numbers on a tyre sidewall mean?

The collection of markings on the side of a tyre can be as puzzling and troubling as any film made by David Lynch.

But we need to understand the gobbledegook. Tyres are critical to safety and comfort, as everyone knows (hopefully).

It’s important to understand what the tyre size information represents, for instance, to find the right replacement tyre at the appropriate juncture.

Yet even those of us who can recite power and torque figures, or fuel economy numbers, might be hard-pressed to remember the type of tyres that are fitted to their car. So let’s address that...

Who makes the tyres on your car? It’s easy to find, because it’s usually a single brand name in huge, bold lettering that can be seen from the outer suburbs.

The better-known names include Bridgestone, Hankook, Goodyear, Pirelli, Yokohama, BF Goodrich, Continental, Kumho, Toyo, Dunlop, Firestone and Michelin.

There’s a host of lesser names as well.

Again, this is more about marketing than information that will make your life better. The lettering of the tyre name is scarcely smaller in size than the brand, and includes words like: ‘Potenza’, ‘Pilot’, ‘Cinturato’, ‘Advan’, ‘Proxes’, ‘Ventus’, ‘Monza’ and ‘Dueler’.

There are usually secondary titles to distinguish slightly different tyre compounds or tread patterns for different purposes. So the Michelin Pilot tyre is available in Sport 2, 3, 4 and 5, and there are numerous variations on the Dunlop Grandtrek theme for SUVs and 4WDs, including ST20, AT22, PT3... etc.

Some truck or off-road tyres exclusively rely on imperial measurements (in old-fashioned inches), but most tyres these days are measured in metric (millimetres), percentage and the wheel diameter in inches.

Here are some examples of typical passenger car tyre sizes:

195/60R15 – as pictured, spare wheel for small passenger car
205/55R16 – typical tyre for small passenger cars and light-duty SUVs
225/55R18 – typical tyre for medium SUVs
225/45R18 – typical tyre for a mid-size prestige sedan

This is the standard format for most road-going tyres these days: three digits, followed by a slash (‘/’), two more digits, the letter ‘R’ and two more digits.

The first three digits refer to the tyre’s width, measured in millimetres. Narrower tyres can measure as little as 145mm for light hatches, and really wide tyres measuring up to 355mm are the sort you’d fit to the rear of a high-performance sports car, like a Porsche.

The two-digit number following the slash is the profile of the tyre, which is a percentage of the width to indicate how low the sidewall profile is, relative to the tyre’s width.

A smaller number (50 series or less) indicates a lower profile tyre, providing reduced sidewall flex for better handling, road-holding and steering, but usually worse ride comfort.

Higher-profile tyres are more common on family cars, SUVs, off-road vehicles and light-commercial vehicles.

‘R’ is for ‘Radial’, the tyre construction. In some cases, the ‘R’ will be preceded by a ‘Z’, which indicates a high-performance tyre capable of managing speeds in excess of 240km/h.

The final two digits are the wheel diameter, measured in inches. Wheels, like infotainment screens but unlike tyres, have never made the jump from imperial to metric measurements, which is obvious in this hybrid format.

Following the tyre’s size on the sidewall is an alphanumeric component comprising two digits – the load index – and a single letter, which is the tyre’s speed rating. It might be ‘91V’, as an example.

Load indices range from 70 (for a tyre bearing a maximum weight capacity of 335kg) all the way up to 126 (1700kg maximum weight). The increments are 10kg initially, but rising progressively to 50kg at the upper end of the scale.

The speed ratings run from ‘L’ (up to 120km/h) to ‘V’ (up to 240km/h) in 10km/h increments, and excluding an ‘O’ rating – presumably as that letter could be easily mistaken for zero (‘0’).

Above ‘V’ there are three additional ratings. ‘W’ denotes tyres considered safe up to 270km/h and ‘Y’ for up to 300km/h.

As mentioned already, ‘Z’ tyres are rated for speeds above 240km/h, but the ‘Z’ rating has been largely superseded by the later ‘W’ and ‘Y’ ratings.

A tyre measurement in the format 30/10.5R15 or 30x10.5R15 is a truck or off-road tyre. The two initial numbers indicate the tyre’s total diameter, measured in inches across the whole tyre, including the centre for the steel or alloy wheel.

One inch equals 2.54cm, so the overall measurement of the tyre’s diameter from top to bottom, when fitted to the vehicle is about 762mm.

The measurement after the slash (‘/’) is the width of the tyre in inches. In the example given, 10.5, the tyre is 10.5 inches across the tread, or 266mm. The letter ‘R’ indicates that the tyre is one of ‘radial’ construction, rather than the cross-ply tyres frequently fitted to trucks in the past.

And the two numbers following the ‘R’ indicate the diameter of the wheel to which the tyre is fitted. In this case, the wheel measures 15 inches across the hub – from top to bottom when the wheel and tyre are in place, fitted to the vehicle.

Any tyre with a 50-series profile or higher will deliver a more comfortable ride.

A lower profile results in reduced flex in the tyre’s sidewall, for enhanced stability and grip.

Tyres narrower than 175mm are eco-friendly tyres.

Tyres wider than 225mm offer a larger contact patch for braking, cornering and acceleration.

A lower-profile on a wider tyre may result in roughly the same level of sidewall flex as a higher-profile on a narrower tyre.

Most spare tyres follow the same general format of dimensions and type as the road wheels, but not temporary-use tyres, which are intended to free up space in the boot without adding weight.

The size displayed usually reflects the very narrow dimensions and relatively high profile. An example would be T125/80R16. Note the leading ‘T’ (for ‘temporary’).

This tyre is only 125mm wide, which is very narrow by the standards of modern tyres, and the profile is 80 per cent, which is quite high for passenger cars built within the last 30 years. It’s still a radial tyre, however, made for a 16-inch wheel.

Tyres carry a four-digit figure on their sidewalls to indicate the week and year they were manufactured.

In the tyre pictured, the ‘date’ or manufacturing batch is ‘1205’, indicating the tyre was manufactured in the 12th week of 2005.

This number provides advice for the vehicle owner. If the tyre concerned is over five years old, it should be replaced, even though the tread might show very little wear.

There are all sorts of extraneous datapoints displayed on the sidewall of a tyre, some of which are required by law. To illustrate, ‘RFT’ indicates the tyre is a ‘run-flat’ type.

There are two US patent numbers on the spare tyre pictured here, along with explicitly stated European Union tyre approval and noise approval compliance numbers.

The tyre’s construction is outlined in full, as are maximum permissible weights and pressures, in accordance with US and Canadian legislation.

We’re told that the spare is a tubeless type and it’s made in Germany. Just to help out the tyre fitter, the English word ‘Outside’ is displayed in raised lettering on the sidewall that is supposed to be visible when fitted to the car.

It’s also joined by the translation in various European languages, including German (‘Aussenseite’) and French (‘Cote Exterieur’). These are probably required by EU mandate.

And that is a brief explanation of almost everything there is to know about your tyres. It’s not an exhaustive list, but it does cover the main points.

Now, when a tyre retailer recommends a ‘+1 upgrade’ with a 35-series tyre, you can safely reply: “Mate, do I look like a masochist with cash to burn?”