You could be forgiven for wondering if they make the numbers and letters on the side of your tyres deliberately confounding. Unless you know what you’re looking at, it may as well be written in hieroglyphics.
Nevertheless, the confusion around decoding what these mystery digits mean, deliberate or incidental, can’t take away from the fact that modern tyres are pretty darn amazing.
Not that long ago, they had cotton plies, belts that weren’t far removed from rebar and rubber compounds that came from a vat with “one-size-fits-all” emblazoned on the side. With a few technological advancements and some solid engineering, however, new tyres grip harder, last longer and are, in a word, better.
The evolution of tyres and why they’re the best they’ve ever been
The tyre journey, from leather bands wrapped around wooden wheels to the introduction of the first pneumatic (air-filled) tyres to the bias ply vs radial battle of the 1940s (radials won), has been long and filled with technological advancements. The results: longer tyre life, higher grip levels and less environmental impact.
In the past couple of decades, most of these advancements have come from tweaks rather than dramatic redesigns, but the advantages are still profound.
From a safety standpoint, the introduction of run-flats has been a bit of a game-changer, as has optimised tread design and recycling of old tyres, reducing rolling resistance (fuel usage) and generally just being a bit of a win for Mother Nature.
The modern tyre game continues to tweak the manufacturing process. When the liquid tyre compound is being mixed up it’s not just rubber in the Kitchenaid; up to five different rubber compounds are used, along with zinc-oxide added to the sulphur (rather than using sulphur on its own) during the vulcanisation process.
Stabilisers, including silica and graphene, are also thrown in during specific periods of heating and cooling, where the tyres are cooled and heated at different temperatures to elicit different softening/hardening on specific parts of the carcass. This means they can be softer and grippier when turned into a corner, yet harder when going straight, resulting in significantly increased traction and tyre life.
Basically, thanks to such chemical wizardry, modern tyres are more durable, have higher tensile strength, have higher heat capacity and basically do everything but fill out your tax return for you.
What the sidewall markings actually mean
When you take a look at your sidewalls, you’ll notice the tyre brand name often followed by a model name. For example: Pirelli (brand) P-Zero (model). A little further around, you’ll find a series of numbers, which are the tyre size, followed by the load rating and speed symbol.
For example – 225/45R17 94Y
To the untrained eye, the tyre size is an absolute dog’s breakfast. The first number is metric, the second is a percentage and the third is in imperial. Let’s break it down anyway:
225 = the width of the tyre when viewed directly form the front. In this case, it’s 225 millimetres wide.
45 = the aspect ratio, or the percentage of the tyre’s width (225mm), which is the height of the sidewall. So, in this case, the height of the sidewall is 225 x 0.45 = 101mm (rounded to the nearest millimetre).
R17 = the rim diameter in inches. This tyre is designed to be mounted on a 17in rim.
94 = the load index. This is the amount of weight, in kilograms, a tyre can handle. In this case, a load index of 94 means it can handle 670kg.
Y = the speed symbol, aka how fast this tyre can safely go without delaminating or send you careening into a bus full of nuns. Again, this is a fairly high rating, with Y = 300km/h.
You’ll also find the manufacturer’s recommended operating pressure, which should be seen as a rough guide rather than a hard and fast rule. Tyre pressure should really be determined on vehicle weight and loads as well as overall usage, not just a number the bloke on the mould machine threw on there for funsies.
Is this rating and measurement system needlessly complicated and annoying to decipher? Yes. Could they re-jig it so it’s all in metric, or imperial (some American brands do this), or easily read so you can determine the tyre size, load rating and maximum speed quickly and easily? Also, yes. But they don’t.
We don’t know why, to be honest, but at least now you know what all those numbers mean.
In the past 15 years as an automotive journo and 35-plus years of inveterate car-guy antics, Dex has worked across numerous titles and has even occupied the occasional editor’s chair when nobody was watching. He spends his downtime doing engine swaps (plural) on the nature strip out front and also once ripped a handbrake spin into a perfect car park. His parents remain indifferent.
