What the sidewall is actually telling you

A tyre is a document that almost nobody reads. Every fact a regulator wants a driver to have is moulded into the sidewall, in a code with no glossary beside it, and the one figure that matters most is not on the tyre anywhere.

The words on the sidewall, as the instruments define them

Load-capacity index. UN Regulation No. 30 defines it as a number associated to the reference mass a tyre can carry when operated in conformity with requirements governing utilization specified by the manufacturer.

Speed category symbol. The same regulation defines the speed category as the maximum speed which the tyre can sustain, expressed by a speed category symbol. The letter is a ceiling, not a compliment.

Tread-wear indicators. Defined in Regulation No. 30 as the projections within the tread grooves designed to give a visual indication of the degree of wear of the tread. Spain’s traffic authority describes the same object from the driver’s side: a raised block sitting on the base of the channels of the tread.

M+S, and the alpine symbol. Regulation No. 30 defines a snow tyre as one whose tread pattern and whose structure are primarily designed to ensure in mud and fresh or melting snow a performance better than that of an ordinary road-type tyre, and requires the inscription M+S, M.S or M&S. The alpine symbol is a different kind of claim: the European labelling regulation admits it only for a tyre that satisfies the minimum snow grip index values set out in UN Regulation No. 117, and keeps a separate ice grip pictogram for tyres meeting ISO 19447. Design intent and a passed test are not the same mark, and they sit millimetres apart on the same sidewall.

The date of manufacture. Regulation No. 30 requires it as a group of four digits, the first two showing the week and the last two the year. The United States rule governs the whole string: a tyre identification number of thirteen symbols, closing with a date code of four numerical symbols identifying the week and the year. A manufacturer’s handbook supplies the worked example — a code ending 1626 is a tyre made in the sixteenth week of 2026.

The pressure figure. The number moulded into the sidewall is, in the manufacturer’s own words, the greatest amount of air pressure that should be put in the tyre. It is the tyre’s ceiling, not your car’s figure, and inflating to it is a misreading rather than a precaution.

Where the pressure actually lives

In the United States, federal standard 110 requires a placard headed Tire and Loading Information, permanently affixed to the driver’s side B-pillar, showing the recommended cold inflation pressure for the front, rear and spare tyres, the original tyre sizes, and a vehicle capacity weight: the combined weight of occupants and cargo should never exceed the stated figure. Spain’s DGT lists the same figure’s usual hiding places — the glovebox, the inside of the fuel flap, the handbook, the inside of the door — and says to check monthly, with a calibrated gauge, cold.

Two ceilings, then, belonging to different objects: the load index caps what one tyre may carry, the capacity weight what the car may carry in occupants and cargo together. A car loaded to the second is a different car to inflate, and the figure for that condition is in the handbook the placard refers you to.

Tread depth: the groove, the band and the country

British law states the requirement as a geometry, not a number: the grooves of the tread pattern must be at least 1.6 mm deep throughout a continuous band comprising the central three-quarters of the breadth of tread and round the entire outer circumference of the tyre. Both halves do work: a tyre worn on one shoulder can hold 1.6 mm in the middle and still be legal, and a tyre that is legal in front of you and bald behind you is not. For vehicles outside the car and light-goods classes the same regulation drops to 1 mm. The inspection manual adds the word the law leaves implicit — the depth is read in the primary grooves.

The United States has no single passenger-car figure; the minimum is set by each state. California’s is worth quoting because it does not match the number people repeat: one thirty-second of an inch tread depth in any two adjacent grooves at any location, rising to four thirty-seconds on the steering axle of a commercial vehicle. It also legislates where not to measure: at tie bars, humps or fillets. Two adjacent grooves at any location is a different test from a continuous band, and a tyre can pass one and fail the other.

The British inspection manual fails a tyre for reasons unrelated to depth — a cut over 25 mm or ten per cent of the section width, a lump, bulge or tear caused by separation of the structure, any ply or cord exposed — and for a load capacity or speed rating below what the vehicle requires. Fitting a cheaper tyre with a lower index is a defect, not a saving.

Age, and the one date the road will never give you

Nothing in a drive reports the age of the rubber. Only the four digits do.

Britain has turned that reading into a rule for part of the fleet only. A tyre is unsuitable for use if the week of manufacture marked on its sidewall in accordance with ECE Regulation 30 or 54 falls more than ten years before the date on which the vehicle is used on the road — on the front axles of buses, on single wheels on any axle of a minibus, and on the front axles of goods vehicles over 3,500 kg. An illegible date marking is itself a defect under the same rule, and vehicles of historical interest used for non-commercial purposes are exempted.

There is no equivalent age rule for an ordinary car in that instrument or in the others read here. The regulators that have legislated on tyre age did it for heavy vehicles and for the axle that steers them, which is narrower than the advice that circulates and is what we can source.

Mixing, and the question we cannot answer

Regulation 26 is unusually literal. Pneumatic tyres of different types of structure shall not be fitted to the same axle. A two-axle vehicle may not carry a diagonal-ply or bias-belted tyre on its rear axle if a radial-ply tyre is fitted on its front axle. Tyres on the steerable axles, and on the driven axles, must all be of the same type of structure. A temporary-use spare is exempt from all of it, unless the car is driven above 50 mph. The inspection manual treats different sizes on one axle and different structure on one axle as major defects, while stating plainly that run-flat and conventional tyres can be mixed on the same axle although this is not recommended.

Every one of those rules is about construction. None is about tread depth, and none says which axle a newer pair belongs on. That question is answered with great confidence all over the internet, and we did not find a primary instrument answering it. What exists for your car is the instruction in its own handbook.

Run-flats, the spare and the bottle

A run-flat is defined by what it must survive. Regulation No. 64 defines one as a tyre provided with technical solutions allowing it to supply the vehicle with basic tyre functions at least at a speed of 80 km/h and a distance of 80 km when operating in flat tyre running mode. Those figures are a floor for type approval, so the number for your car is whatever its own handbook says. And basic tyre functions for 80 km is a definition of getting home, not of continuing.

The temporary spare carries its own limit on its face: the same regulation requires an 80 km/h maximum speed warning symbol permanently displayed on the outer face of the wheel.

The sealant bottle is the least understood, and the instruction here is a manufacturer’s rather than a regulator’s. Kia’s handbook says the kit seals most punctures caused by nails or similar objects, that larger punctures or sidewall damage cannot be sealed completely, and that only areas within the tread region can be sealed. After use it sets a maximum speed of 80 km/h and a distance of up to 200 km, tells the owner to have the tyre repaired or replaced as soon as possible, and warns against sealant used past the expiry date on the container. A bottle that has sat in the boot since the car was new may already be useless.

For a fleet, the vocabulary becomes a specification

A company buying tyres is choosing a load index and a speed symbol, and those two figures make a purchase order checkable against the vehicle. A company fitting them is recording a four-digit date code, which is the only way an age policy can be audited or a supplier’s older stock made visible. Most fleet inspection sheets are written in the language of the tyre shop; they should be written in this one.

What we cannot tell you

We cannot tell you the correct pressure for your car, because it is specified per vehicle and per load. We cannot give you a tread-depth figure for the United States, because that minimum belongs to each state and only California’s was read here. Nor can we tell you how an insurer will treat a worn tyre after a crash. A disagreement about whether tyre condition was properly weighed is a complaint about how the claim was handled, and the supervisor named in the data below is the body that takes it.

Your own tyre, in these words

Find the four digits and work out how old the rubber is before anything else, because no journey will tell you. Check the load index and the speed symbol against the placard on the door pillar rather than against what was fitted last time, and take the pressure off that placard or the handbook, never off the sidewall, with a gauge, cold. Look for the tread-wear indicators in the primary grooves, and at the shoulders as well as the centre, because the rule is a band and not a spot. Then ask a construction question about the other three rather than a wear question: same structure on the axle, same sizes, and if one is a temporary spare, the speed on its face is the speed for the whole car.

None of that is a service. It is a reading, and it takes about as long as filling the washer bottle.