Tyre age: what the DOT code tells you

On the sidewall sits a block beginning with “DOT”. What follows is a string of letters and digits: the plant, a size key, an internal code — and at the end, usually in its own recessed frame, four digits. The first two give the calendar week, the last two the year. “3419” means built in the thirty-fourth week of 2019.

Two things about that matter in practice. First, the four-digit form is the one in use since the turn of the century; older tyres carry three digits. Anyone who finds a three-digit field on a trailer, a spare or a long-parked car is holding a part whose age is no longer measured in years.

Second, the complete marking is often moulded on one side of the tyre only. The other side carries the same block without the date field. So if all you find on the car is a shortened code, the tyre is not undated — you are looking at the wrong side, and that is often the inner one. With the steering on full lock and a torch you can reach the front wheels; at the rear it takes a lift or patience.

A tread compound is not one substance but a mixture of rubber, carbon black or silica, plasticisers and anti-ageing agents. Those agents migrate slowly to the surface, where oxygen and ozone consume them. That is their job — but the supply is finite, and it shrinks whether or not the car ever leaves the garage.

Once it is gone, oxidation attacks the polymer network. The rubber hardens and loses elasticity, and the stresses that flexing creates on every revolution have nowhere to go. It shows up as fine crazing — first at the base of the tread grooves and in the sidewall zone that bends on every turn.

Harder means less grip, and not evenly so: the loss hits wet and cold roads first, because that is where the compound’s ability to deform does the actual work. On dry, warm tarmac an old tyre feels unremarkable for a long time. That is precisely what makes it treacherous.

Tread depth measures how much material has worn away. Age measures how long the compound has had to change. The two are unrelated. A car driven little each year has deep tread on old rubber; a much-driven one has the opposite. Read only one of the two clocks and you know half the story.

The bars at the base of the main grooves, marked “TWI” on the sidewall, show the point at which the tread counts as worn out. Which figure applies, and whether winter tyres are held to a different one than summer tyres, is set by the law where the car is registered — not by the tyre maker and not by this article.

For age there is no such mark in the rubber. When a tyre counts as too old is answered by the vehicle maker’s recommendation and by the rules where the car is registered — often stricter for trailers and caravans than for passenger cars. If you need a figure, that is where you get it, not from a guide.

A tyre that has never been fitted is new. That says nothing about when it was built. Between production and sale lie a warehouse, transport and a shelf, and in a slow-moving size that journey can take years. If you are buying a set, you are entitled to see the DOT code before it goes on the rim — afterwards the question is academic.

Storage plays its part. Tyres kept cool, dark, dry and unloaded age more slowly than ones that stood in sunlight or beside a machine with a brushed electric motor: its sparking produces ozone, and ozone is the most aggressive part of the air as far as a rubber compound is concerned. Fuel and solvent vapours do not belong in the same room either.

For your own second set the rule is simple: store them upright and turn them occasionally when they are off the rim, stacked flat or hung when they are on it. And with a note of which axle and which side they ran on — otherwise that information is lost at fitting time, and with it any chance of reading the wear.

Next to the date comes the size designation, and it is put together from mixed units: the width in millimetres, then the ratio of sidewall height to width as a percentage of that width, a letter for the construction — “R” for radial — and finally the rim diameter in inches. Sidewall height is therefore not a fixed value but depends on the width: the same percentage gives a taller sidewall on a wider tyre.

Then come the load index and the speed symbol. Both are table values: the index stands for a load per tyre, the letter for a permitted maximum speed. What a given car requires as a minimum is in its own documents, and a tyre may exceed those values but not fall short of them.

Change the size and you change the rolling circumference, the speedometer reading and, depending on the car, the clearance in the wheel arch. Whether a combination is permitted is decided by the vehicle’s documents, not by how it looks in the arch. On a used car the comparison is worth making: the size fitted and the size on the papers should agree.

A winter tread compound is designed to stay soft below a certain temperature. That is exactly the property ageing takes away first. An old winter tyre with plenty of tread can feel perfectly fine in summer and deliver far less in February than its tread pattern suggests. That is the situation in which age genuinely costs you something.

Two markings on the sidewall address this, and they do not mean the same thing. “M+S” is the maker’s own statement about the tread pattern, with no prescribed test behind it. The mountain-and-snowflake symbol is granted only when the tyre has passed a standardised snow traction test. Which marking is required where is again a matter of local law.

All-season tyres carry the same logic in a single compound: they are a compromise between two temperature ranges, and an ageing compromise loses at both ends. Anyone taking the car into the mountains in winter should therefore check the date on an all-season tyre as carefully as the tread.

A bulge in the sidewall is not a cosmetic flaw. It appears when cords in the casing tear against a kerb; air pressure then pushes the weakened spot outwards. The tyre may hold for weeks afterwards and then fail without warning. It gets replaced, not monitored.

Fine, net-like cracking at the base of the grooves and in the sidewall flex zone is the picture of ageing. A single crack says little, an area of crazing says a lot. Cuts with cord showing through, and places where the tyre has worn down to the casing on one side, are not bargaining items but a reason not to drive the car away yourself.

Uneven wear across the width also points to something else. Worn unequally on the inside and outside means toe or camber; a sawtooth pattern on the tread blocks usually means suspension or chronically low pressure. Replace the tyres and leave the cause in place, and you buy the same wear a second time.

“New tyres” in a listing is a claim you can check in two minutes. You read four date fields, compare tread and type on each axle, and look at the inner sidewalls. What comes out is either confirmation — in which case the seller is right and the price may reflect it — or an item for the list.

An item whose size is known, at that. A set of tyres in a common size can be quoted over the phone, fitting and balancing included. A written quote moves a price; the remark that the tyres are “a bit old now” does not.

One question belongs before the test drive rather than after it: when was the pressure last checked? A tyre run for a long time underinflated has lost material at the shoulders and taken heat into the sidewall that you cannot see. The date says nothing about that — the wear pattern does.