Tyres: sidewall markings, tread patterns, and what pressure actually changes

Everything a car does — accelerating, braking, steering, staying on the road — happens through four patches of rubber roughly the size of a hand. Nothing else on the car is asked to do so much with so little, and nothing else is so routinely bought on price and fitted without a second thought.

This is a merge of six pieces from the archive. The best of them is careful and useful; the largest is 2,250 words recommending something that does almost nothing. Both are dealt with below.

Reading the sidewall

A marking like 205/55 R16 91V is four separate facts.

Two things people miss. The aspect ratio is a percentage, not a measurement, so a 225/55 has a taller sidewall than a 205/55 despite the same number. And the load index and speed rating are not optional: they are chosen for the car's weight and capability, and fitting below them is both unsafe and, in most places, illegal.

And there is a date on there too. A four-digit code in an oval — 3419 means the 34th week of 2019. Rubber hardens and cracks with age whether the tyre is used or not, so a tyre with full tread can still be finished: six years is where manufacturers start recommending replacement, ten is where it becomes difficult to defend. A car that does 3,000 km a year is exactly the car this catches, along with the spare that has been under the boot floor since the car was new.

Pressure, and the number almost everyone reads off the wrong thing

The archive is right that pressure affects fuel consumption, ride, braking, noise, steering and grip. What none of its pieces say is where to get the number.

The figure on the tyre sidewall is not the recommended pressure. It is the maximum the casing can take at maximum load. The pressure to use is on the placard in the driver's door aperture, in the fuel filler flap, or in the handbook — and it is set for that car, often with different figures front and rear and a higher set for a full load.

Inflating to the sidewall maximum gives a harsh ride, a smaller contact patch, less grip and more risk of impact damage. It is a common and confident mistake.

Check them cold — before driving, or at least three hours after — because a warm tyre reads several psi high and "correcting" that leaves the car underinflated when it matters. Once a month, and before a long trip. And do the spare, which is the one tyre guaranteed to be flat on the day you need it.

Nitrogen: the archive's largest tyre article is about something that barely matters

One merged piece runs to 2,250 words and 143,000 views on filling tyres with nitrogen, and lists as benefits: lower fuel consumption, longer tyre life, more safety, better handling, more grip, a softer ride, and no need to keep checking the pressures.

Every one of those is a benefit of correct tyre pressure, which you get from air, for nothing, by checking it. The nitrogen is not doing them.

What is actually true is much smaller, and worth stating plainly because the claims are so widespread:

• Ordinary air is already about 78 per cent nitrogen. Filling with nitrogen is going from 78 per cent to perhaps 95.
• Nitrogen does permeate the rubber slightly more slowly. Measured over a year, the difference is around one psi — real, and far less than the pressure you lose by never checking.
• It is dry, so there is less moisture inside to drive corrosion and to make pressure swing with temperature. On aircraft and racing cars, where that matters, it is used for exactly that reason.
• Top up with air once and the benefit is diluted away, which is what happens to almost every road car.

So: harmless, occasionally convenient, and not a substitute for a pressure gauge. The last claimed benefit in that list — that you no longer need to check the pressures — is the one that makes the article worth correcting rather than just trimming, because acting on it is how a tyre ends up dangerously soft.

Changing the size, which the best of the sources is good on

One merged piece is a careful argument against fitting wider, lower-profile tyres for appearance, and it deserves to survive because it is right and unfashionable.

A lower profile does improve steering response and cornering grip — less sidewall means less flex, so the tyre reacts more directly. That is why manufacturers have moved that way.

And it costs you the things sidewall was doing. Ride comfort, first and obviously. But also impact protection: the sidewall is the suspension's last few millimetres of travel, and a low-profile tyre transmits a pothole straight into the wheel rim and the suspension components. Bent rims, split tyres and worn bushes follow.

The source's central point is the one to keep: the manufacturer chose a size and then tuned the suspension, steering and stability control around it. A small deviation is unimportant; a much wider tyre with a much shorter sidewall is a change to a system that was engineered as a whole, and the bill arrives as ride quality, steering weight and chassis wear rather than as an obvious fault.

One technical constraint the source does not give. The rolling circumference must stay close to standard, because the speedometer, the ABS wheel speed sensors and the stability control all assume it — and on a four-wheel-drive car a mismatch is actively destructive, as four-wheel drive explains. Going wider usually means going lower to compensate, which is why the two changes arrive together.

Tread, and what it is for

Tread is not for grip. On a dry road a slick has more grip than any patterned tyre, which is why racing cars use them. Tread exists to give water somewhere to go.

Which makes tread depth a wet-weather number rather than a general one. The legal minimum is 1.6 mm across the central three-quarters in most of Europe, and the performance falls off well before that — braking distances and aquaplaning resistance in the wet degrade sharply below about 3 mm, which is why many manufacturers suggest replacing there rather than at the legal limit.

Three pattern types, and the fitting rules that go with them:

• Symmetric — fits any way round, rotates freely.
• Directional — a V-shaped pattern designed to throw water forward and out. Must be fitted the right way round, marked by an arrow on the sidewall. Fitted backwards it is measurably worse in the wet, and this happens more often than it should.
• Asymmetric — different tread on the inner and outer halves. Marked outside and inside, and it matters just as much.

And the summer-versus-winter difference is not about snow. It is a rubber compound: below about 7 °C a summer compound hardens and loses grip on a cold dry road as well as a wet one, while a winter compound stays pliable. That threshold is the useful fact, and it is why winter tyres are worth having in a climate that has cold mornings but rarely snows.

Where to put new ones

If you are buying two rather than four, the new pair goes on the rear — whichever wheels drive the car.

This is counter-intuitive on a front-wheel-drive car, where the front tyres do the steering, most of the braking and all of the driving, and it is still correct. Worn tyres at the front cause understeer, which is stable and which a driver feels and corrects. Worn tyres at the rear cause the back of the car to let go first in the wet, and that is a spin. Put the grip where losing it is unrecoverable.

What tyres tell you about the rest of the car

A tyre keeps a record of everything acting on it, which makes the tread a diagnostic surface as much as a wearing one — one edge worn is camber, a feathered sawtooth is toe, scalloped dips are dampers. That is set out in full in wheel alignment and springs and dampers, and it is the reason to look at all four properly rather than glance at the tread depth.

One more that belongs here rather than there: a bulge in the sidewall means the casing cords inside have broken, usually from a pothole or a kerb. It does not heal, it cannot be repaired, and it is the failure that becomes a blowout. Replace it.

Video guides

Video, for the parts of this that are easier watched than read.