How to read a tire date code on the sidewall

The date is the last four digits of the long DOT string moulded into the sidewall: two digits for the week, two for the year. 3524 is the thirty-fifth week of 2024. Reading it takes ten seconds. Finding it is what goes wrong, because on a great many tyres the sidewall facing you carries no date at all, and no amount of squinting will produce one.

Count backwards from the end

The rule is 49 CFR 574.5, NHTSA’s tire identification and recordkeeping regulation, and it is unusually direct: “The date code, consisting of four numerical symbols, is the final group.” The first two symbols identify the week, using 01 for the first full calendar week of the year. The third and fourth are the last two digits of the year. The regulation supplies its own example: “0109 means the tire was manufactured in the first full calendar week of 2009.”

Continental reads it the same way, noting that the week digits “range from ‘01’ to ‘53’” and that DOT XXXXXXX 4521 “was manufactured in the 45th week of 2021”.

Count from the end, not from a fixed position, because what sits in front of the date varies. Since April 2025 the regulation requires thirteen symbols: a three-symbol plant code assigned by NHTSA, a six-symbol manufacturer’s code, then the date. The older format — permitted until 13 April 2025, and still on nearly every tyre on the road — was shorter and variable: a two-symbol plant code, up to two symbols for the tyre size, up to four optional symbols describing the tyre or naming a brand-name owner, then the same four digits. The DOT letters are not part of the number: under 574.5(e) that symbol certifies compliance with a Federal Motor Vehicle Safety Standard.

Why half the time there is no date to find

The trap is written into a different regulation: FMVSS No. 139, at 49 CFR 571.139, which covers new radial tyres for vehicles rated at 10,000 pounds or less and built after 1975.

For tyres made on or after 1 September 2009, S5.5.1 requires the full tire identification number “on the intended outboard sidewall of the tire”. On the other sidewall, the manufacturer may instead mould “a partial tire identification number, containing all characters in the tire identification number, except for the date code and, at the discretion of the manufacturer, any optional code.”

The date code is precisely what the partial number omits. Everything else is there — plant code, manufacturer’s code, the DOT letters — which is why the inner face looks complete until you look for four digits at the end of it.

Before September 2009 the rule was looser still: the full number went on “a sidewall”, either one. That is the origin of the advice that the date faces inward — often true on older tyres, no longer the general case, though a symmetric tyre with no designated outboard side can still be mounted either way round.

So a string ending without a date is not a defective marking. It is the wrong face. Turn the steering to full lock and the inner sidewalls of the front tyres come into view; for the rears there is no way round crouching with a torch.

Three digits means the decade is missing

Tyres built before the format changed carry three digits: two for the week and one for the year. That digit does not tell you the decade, so 328 is the thirty-second week of some year ending in eight, and the code cannot distinguish between them.

NHTSA fixed this deliberately. Its own account of the change, in the Federal Register of 1 December 2000, says the agency “amended NHTSA’s tire identification and recordkeeping regulation in 1999 to require the date of manufacture to be expressed in four digits, instead of the previously required three, so that consumers would be able to determine the decade of manufacture of their tires.”

What the regulation requires, and what it does not

Part 574 requires the date to be there, permanently moulded into the sidewall — or, at the manufacturer’s option under 574.5(d)(1), laser etched within 24 hours of the tyre leaving the mould. It exists so a recalled production batch can be identified.

It does not say a tyre expires. There is nothing in Part 574 about age, service life or replacement — that advice comes entirely from the manufacturers, and they do not agree.

The manufacturers disagree, by four years

Michelin recommends “replacing tires ten years after their date of manufacture, regardless of remaining tread depth or apparent condition”, adds that “this recommendation includes spare tires”, and asks for a professional inspection at least once a year “after five years of service”.

Bridgestone lands in the same place: “After five years, an inspection is essential to determine if a tire is still road-worthy,” and tyres “manufactured 10 years prior (or longer)” should come out of service — “Same goes for the spare.”

Continental likewise “recommends that all tires that are over 10 years old (as shown on the tire’s date stamp) should be replaced with new tires.”

Goodyear sets a materially different limit, and starts the clock somewhere else. It recommends that “any tire in service … 6 years or more be replaced even if such tire appears serviceable and even if it has not reached the legal treadwear limit”, falling back to the DOT code only “if you are unable to determine the date a tire was first placed in service”. Goodyear is also candid about the evidence: “no widely accepted scientific research exists to show that chronological aging alone adversely affects a tire”, only that a longer time in service means more exposure to adverse conditions.

Four years apart, then, and disagreeing about whether the clock starts at manufacture or at fitting. All four also say age is not the whole story — Michelin that “tire age alone does not determine whether a tire is serviceable”, Bridgestone that “the age of a tire is not the only indicator”. None of them disputes that the date code is the number you read to have the conversation at all.

Check before fitting, not after

New tyres sit in warehouses. Continental states the trade practice plainly: “A properly stored, unused tire up to 5 years old can be sold as a new tire and placed into normal service if no local legal regulations are opposed to it,” noting that “the warranty period begins with the purchase date of the tire.”

Set that against Continental’s own ten-year limit and the arithmetic is uncomfortable: a tyre sold to you as new at five years old has five years of recommended service left, not ten. Under Goodyear’s rule, which runs from installation, shelf time costs you nothing; under Michelin’s and Bridgestone’s, it costs you every year of it. Neither the convention nor the date appears on the invoice. So ask to see the code before the tyres go on, and check all four — they need not come from the same batch.

A black code on black rubber is close to the worst case for a phone

Photograph the sidewall, feed it to any text recogniser, and expect a poor result. Three problems stack up.

There is no contrast. Raised black rubber on black rubber: nothing separates the characters by tone, and the only signal is the shadow a raised edge casts. A flash pointed straight at it removes that shadow, which is why on-camera flash makes this job worse. Light it obliquely — a torch held off to one side, low sun behind your shoulder — so the edges throw shadows across the surface.

The surface curves. The code runs along an arc, so perspective changes within a single string, and engines handle rotation far better than perspective. Square the lens to the patch carrying the code rather than to the wheel, and take one group at a time.

Everything is glossy. Tyre dressing, rain, road film. A specular highlight lands as a burned-out band with nothing recoverable underneath, so move yourself or the light rather than adding more, and wipe the rubber first.

Size compounds all three: when NHTSA added the fourth digit in 1999 it also cut the minimum symbol height to 4 mm. At any real distance or angle that leaves very few pixels of character height, and below roughly ten pixels accuracy collapses. Why OCR gets numbers wrong covers that threshold, and why enlarging the photo afterwards never helps.

The fix is to crop to the four digits and restrict recognition to digits. Our free browser tool for pulling only the digits out of an image does the second part — SwipeScan is our app, and this is that numbers-only mode with the camera removed — so the DOT letters and the plant code cannot come back spliced into the result. Checking a set of four is where the app’s timestamped history earns its place, the same argument as counting stock with a phone camera. iPhone Live Text and Google Lens are free and already on your phone, and on a clean, obliquely-lit sidewall they often just read it.

One limit applies to every tool including ours: a date code has no checksum. A VIN’s ninth character is a mod-11 check digit that makes a misread detectable — see how to read a VIN — and four date digits have no equivalent. The only checks available are weak and free: the week must fall between 01 and 53, and the year cannot be in the future. Past that, a misread 3524 and a correct 3524 look identical, so read it twice.

The short version

• The date is the last four digits of the DOT string: week, then year. 3524 is week 35 of 2024. Count from the end — what precedes it varies in length.
• Only one sidewall carries it. FMVSS No. 139 lets the other side carry a partial number with the date code specifically omitted, so a string ending without four digits means the wrong face.
• Three digits instead of four means no decade. NHTSA required the fourth in 1999 so owners could tell which decade a tyre came from.
• NHTSA requires the date but sets no expiry. Michelin, Bridgestone and Continental say ten years from manufacture; Goodyear says six years in service. All four say age is not the only factor.
• Check the code before fitting, on all four tyres. Continental confirms an unused tyre up to five years old may still be sold as new, which can quietly halve the recommended service life.
• Light it from the side, never straight on; square to the code, not the wheel; crop to the four digits. There is no checksum, so nothing can tell a misread from a correct read.