Tire Aging: DOT Date Codes, the 6-Year Rule, and Why Old Tires Are Dangerous

Reading the DOT Date Code

Every tire sold in the United States has a DOT (Department of Transportation) code molded into the sidewall. This code certifies that the tire meets federal safety standards, and it ends with a four-digit date code that tells you exactly when the tire was manufactured.

The format is WWYY — two digits for the week of manufacture, two digits for the year. A tire with "2319" at the end of its DOT string was made in week 23 of 2019 — mid-June 2019. A tire with "0124" was made in week 1 of 2024 — first week of January 2024.

Before 2000, the date code was only three digits — two for the week and one for the decade year (e.g., "239" meant week 23 of the 9th year of the decade, which could be 1979 or 1989 depending on other context). If you encounter a three-digit date code, the tire is over 25 years old and should have been replaced long ago.

Important: the full DOT string including the date code may only be molded on one sidewall. If the tire is mounted on the rim with the date side facing inward, you need to look at the inner sidewall to find it. On a lift with the wheels off, this is easy. On the ground, it may require a light and patience.

How Tires Age: The Chemistry

Rubber is a polymer — a long chain molecule that gives it elasticity and strength. Over time, several processes break down that polymer structure:

Oxidation: Oxygen from the air penetrates the rubber and attacks the polymer chains, causing them to cross-link excessively (the rubber becomes hard and brittle) or break apart (the rubber becomes weak). This is the primary cause of tire aging. It happens at the surface first, then works inward over years.

UV exposure: Ultraviolet radiation from sunlight accelerates oxidation, particularly on the sidewall. Tires that park in direct sunlight year-round age faster than tires kept in a garage. Ozone also attacks rubber, and areas with high ambient ozone (some urban environments, near certain industrial equipment) can accelerate sidewall cracking.

Thermal cycling: The repeated heating and cooling as the tire runs and sits causes the rubber compounds to expand and contract. Over thousands of cycles, this fatigues the material and contributes to micro-crack formation.

Tire manufacturers add antidegradants to the rubber compound during manufacturing — these migrate to the surface over time and form a protective layer. Tires that are driven regularly "pump" these antidegradants to the surface through flexing. Tires that sit unused lose this protection faster than tires that are regularly exercised. This is counterintuitive — a tire that never moves ages faster than a tire on a daily driver.

The 6-Year Rule

Multiple vehicle and tire manufacturers recommend replacing tires that are 6 years old, regardless of tread depth. This is not a conservative marketing position designed to sell more tires — it's based on the degradation chemistry described above and on incident data.

The NHTSA has investigated numerous tire failures and blowouts on tires that had adequate tread depth but had exceeded recommended age limits. The failure mode is internal — the rubber between the belts and the body plies degrades, the adhesion between layers fails, and the belts separate from the carcass. This causes a sudden loss of structural integrity — a blowout — often without warning signs visible from the outside.

The 6-year recommendation applies from the date of manufacture (the DOT code), not from the date of purchase. A tire that sat in a warehouse for 2 years before being sold to a customer starts that customer's service life at year 2, not year 0. If you're selling tires, check the date codes on your inventory. If you're installing tires with a manufacture date more than 2–3 years ago, disclose that to the customer.

At 10 years from manufacture, most manufacturers set an absolute replacement requirement — regardless of visual condition, regardless of tread depth. A 12-year-old tire with 8/32 tread is not a good tire. It is a tire waiting to fail.

Dry Rot: Visual Identification

Dry rot is the visible manifestation of rubber oxidation and degradation. What to look for:

Sidewall cracking: Fine hairline cracks that run parallel to the tread (circumferential) or radiate outward from the tread shoulder. Hairline surface cracking is the early stage. Deeper cracks that open up and show underlying layers are the advanced stage. Any crack that is deeper than a surface scratch is a red flag.

Tread groove cracking: Cracks forming in the bottom of the tread grooves. Look carefully — these can be missed in a quick walk-around. Run a light at an angle to the tread to see groove bottoms. Cracks here indicate the degradation is happening at depth, not just on the exposed surface.

Hardened, inflexible rubber: Old tires feel hard and plasticky compared to a new tire that feels slightly tacky and resilient. Press your thumbnail into the sidewall of a suspect tire — a healthy tire resists and springs back. A degraded tire feels stiff and may show a slight indent that holds.

Color change: New tire sidewalls are deep black. Aged tires fade to a lighter gray or brownish black. This is surface oxidation and is normal over time, but rapid fading combined with cracking is a strong indicator of accelerated degradation.

Special Cases: Spares, Classic Cars, Low-Mileage Vehicles

Spare tires are often forgotten in age discussions. The spare in a truck's bed or the compact spare in a trunk ages at the same rate as the mounted tires — faster, in fact, because it's not being regularly flexed to pump antidegradants to the surface. Check the date code on the spare at every major service. A 10-year-old spare is not reliable for emergency highway use.

Classic and collector vehicles are a common source of dangerously old tires. A car driven 500 miles per year will still have deep tread at age 20 — and completely degraded rubber. The owner looks at the tread depth and sees no problem. The tech who knows about tire aging can save someone's life by explaining this clearly and without condescension.

Low-mileage vehicles — second cars, winter cars stored for the summer, vehicles that were garaged for years — present the same issue. Mileage is not a proxy for tire condition. Age is the determining factor for structural integrity. A 3,000-mile tire that is 8 years old is not a better tire than an 8-year-old tire with 40,000 miles. Both are past recommended service life.

Visual Inspection Procedure

At every vehicle inspection:

1. Check tread depth with a gauge at three points per tire — inner, center, outer shoulder.
2. Find the DOT date code on each tire. Write it down. Calculate the age. Flag any tire over 6 years old for customer conversation.
3. Inspect sidewalls on both the visible and inner face (use a flashlight). Note any cracking — describe location and severity.
4. Inspect tread grooves for bottom cracking — angle the light to see groove bottoms clearly.
5. Check for any bubbles or bulges in the sidewall — these indicate internal belt or ply separation and are immediate replacement items regardless of age.
6. Check the spare. Date code and visual condition both.

Document what you find. A tire that is 7 years old with no visible cracking still gets noted and communicated to the customer — they should be monitoring it and planning replacement. A tire with visible deep cracking gets a written recommendation for immediate replacement regardless of age or tread depth. The tread is not the story. The structure is.