Tyre Date Codes, Tread Depth, and When Replacement Is Genuinely Due

Tyres are unusual in the ownership-cost world: the manufacturer has already printed the evidence you need on the product. Age, dimensions, load capacity, speed rating, and the wear limit are all moulded into the rubber. You do not need a technician's opinion to establish the basic facts. You need five minutes and a torch.

That does not make every replacement recommendation wrong. It makes every replacement recommendation checkable, which is a considerably better position than you occupy on most repairs.

Reading the sidewall

Three things are worth finding.

The size and ratings. A string such as 225/60R17 with a load index and speed rating following it. The correct specification for your vehicle is on a placard in the driver's door jamb, along with the manufacturer's recommended cold inflation pressures. Note the placard, not the sidewall, for pressure — the number on the tyre sidewall is a maximum, not a recommendation, and inflating to it is a common error.

The DOT code. A sequence beginning with the letters DOT. The last four digits are the manufacture date: two digits for the week, two for the year. A code ending 3123 means the thirty-first week of 2023. Note that the full code is often moulded on only one side of the tyre, so if you cannot find the date, look at the inboard face.

This matters because rubber ages whether or not it is driven on. Compounds oxidise, and a tyre with generous tread depth can still be old enough to have hardened. Manufacturers and vehicle makers publish their own guidance on service life, and it is stated in years, not miles. It is worth looking up the guidance for your vehicle rather than accepting a shop's rule of thumb, in either direction — a tyre is not automatically finished at an arbitrary birthday, and it is not automatically fine because the tread looks deep.

The treadwear grade. Part of the UTQG marking. Useful for comparing tyres against one another, not for judging the one currently fitted.

Reading the tread

Modern tyres have moulded wear indicator bars sitting in the grooves at the legal minimum depth. When the surrounding tread has worn down level with those bars, the tyre has reached the limit. That is a designed-in, physical, unarguable check that anyone can perform by looking.

The traditional coin test is a reasonable proxy if you cannot see the bars clearly, but the bars are the actual specification and they are already there.

What matters at least as much is how the tyre has worn, because the pattern is diagnostic:

• Both edges worn, centre fine. Chronic underinflation.
• Centre worn, edges fine. Chronic overinflation.
• One edge only. An alignment issue — camber or toe.
• Scalloped or cupped patches. Frequently a worn damper or other suspension component.

That last one is the expensive category, because replacing tyres without fixing the cause simply schedules the same purchase again. If a shop points at cupped wear and recommends only tyres, ask what caused it. If they point at cupped wear and recommend tyres plus suspension work, they may well be right, and that is a legitimate instance of the add-on being real rather than opportunistic.

The arithmetic that shops rarely do out loud

Two questions decide most of the money.

How many? Replacing all four when two are serviceable is the single largest avoidable cost here. Two is often correct — and on a rear-wheel or front-wheel drive vehicle the new pair conventionally goes on the rear axle for stability reasons, regardless of which end drives. There are genuine exceptions: many all-wheel-drive systems specify that circumference differences between axles must stay within a tight tolerance, which can make four the manufacturer's requirement rather than the shop's preference. Ask which situation you are in, and ask them to point at the manual.

What is on the invoice besides tyres? Mounting, balancing, new valve stems, disposal, and often a road-hazard product. As a worked example, assume $180 per tyre and $35 per tyre of fitting-related charges: two tyres is $430, four is $860, and the difference is entirely decided by the answer to the previous question. Substitute your own quoted figures; the structure is what matters.

Alignment deserves its own note. If the wear pattern shows an alignment fault, paying for alignment with new tyres is protecting the purchase. If the wear is even and the vehicle tracks straight, alignment sold reflexively alongside every tyre set is closer to the pattern described in reading a service invoice.

What we would do

Walk round the car with a torch. Find the date codes on all four. Look at the wear bars. Look at the wear pattern. Check the door placard and compare it against your actual cold pressures.

Do that before the conversation, not after it. You will either arrive already knowing the recommendation is correct — in which case you can concentrate on the price and the number of tyres — or you will arrive holding a fact that changes the conversation entirely.

And when the pattern is uneven, treat the tyres as a symptom. Buying new rubber to cover a suspension or alignment fault is one of the more reliably expensive habits in motoring, and it is invisible until the second set wears out the same way. If the underlying repair is significant enough to make you question the vehicle altogether, that is the calculation in the 120,000-mile question.