On the sidewall of every tire, a series of raised characters forms a code that encodes its size, construction, origin, and age. These dot numbers and letters are often misinterpreted or overlooked, yet they are essential for verifying legal service history, determining remaining tire life, and matching replacement tires to vehicle requirements. This guide explains how to read these markings systematically and use them to make safer purchase, maintenance, and replacement decisions.
What the sidewall markings actually mean
The characters molded into the rubber combine standardized codes and optional branding. The U.S. Department of Transportation (DOT) requires a DOT code that traces the plant and inspection date, plus a four-digit build date that indicates the week and year of manufacture. Metric and inch-based size sequences communicate dimensions in different markets, while load index, speed symbol, and construction type define performance limits. Understanding each segment answers basic questions about sizing, age, and intended use.
Key marking types and placement
- DOT code: starts with DOT, identifies manufacturing plant and inspection date.
- Build date: four digits showing week and year of production.
- Size and section: width, aspect ratio, and construction in metric or inch format.
- Load index and speed symbol: numeric code for load capacity, letter for max speed.
- Traction and temperature: UTQG grades for wet traction and heat resistance.
How to read the DOT code and build date
Locate the DOT marking near the edge or center of the sidewall. Immediately after DOT and any plant code, the last four digits represent the week and year of manufacture. For example, 2319 indicates the 23rd week of 2019. You can calculate tire age by subtracting the model year from the current year, which is critical because many regions consider tires over six years old potentially unsafe regardless of tread depth.
Sample DOT and build date decoding
| Marking Segment | Verified Detail | Source Type |
|---|---|---|
| DOT code | Plant location and inspection date | Regulatory standard |
| Last four digits | Week and year of manufacture | Regulatory standard |
| Size sequence | Width, aspect ratio, rim diameter | Metric or inch standard |
| Load index and speed symbol | Carrying capacity and speed rating | Metric standard |
| UTQG grades | Traction and temperature resistance | Uniform Tire Quality Grading |
Decoding the size sequence
Metric size codes list width in millimeters, aspect ratio as a percentage of section width, and rim diameter in inches, separated by slashes and sometimes an optional wheel designation. P-metric codes begin with P for passenger car tires, while LT and ST codes indicate light truck and trailer applications. Inch-based sizes, common on some older North American vehicles, use a similar height-to-width ratio with a diameter specified in inches. Matching these sequences ensures proper fit, load capability, and speed rating for the intended vehicle.
P-metric versus LT and ST formats
- P-metric: P215/65R15 95H, where P signals passenger car use.
- LT metric: LT245/75R16 116/114Q, for light truck applications.
- ST metric: ST245/75R16, for trailers.
- Inch-based: 31x10R15, common on utility and older off-road vehicles.
Verifying tire age and avoiding outdated stock
Tire age affects safety and eligibility for warranty or insurance. Rubber compounds degrade even when a tire sits in storage, and age-related cracking can appear in as little as six years in harsh climates. To verify production date, locate the four-digit build date and compare it to today’s date. For vehicles with pre-owned tires, ask sellers or service centers to disclose the build date, and prefer newer batches when options are comparable. Many manufacturers recommend replacing tires within six to ten years from manufacture, so use the DOT build date as a firm reference point.
Acceptable use thresholds by age
- 0–3 years: Optimal performance and full warranty coverage.
- 4–6 years: Inspect annually for cracking and wear; replace if degradation appears.
- 7–10 years: Many insurers and shops advise replacement; check regulations and manufacturer guidance.
- Over 10 years: High risk of structural issues; not recommended for service.
Comparing load index and speed ratings
The load index is a two- or three-digit code that translates to a maximum load in kilograms supported at a specified pressure. The speed symbol, a single letter, indicates the maximum speed at which the tire can safely carry that load. Matching both codes to the vehicle’s requirements ensures that handling, braking, and durability remain within design limits. Substituting a lower-rated tire can increase risk of overheating, irregular wear, or failure under stress.
| Index | Load (kg) | Speed Symbol | Max speed (km/h) |
|---|---|---|---|
| 91 | 615 | H | 210 |
| 95 | 690 | V | 240 |
| 100 | 800 | W | 270 |
Interpreting traction and temperature grades
UTQG traction grades evaluate wet straight-line braking performance on specified surfaces, with AA being highest and C lowest. Temperature grades describe resistance to heat generation and dissipation, with A indicating excellent resistance, B good, and C acceptable for limited conditions. These grades provide a baseline for comparative shopping, but real-world performance depends on tire pressure, alignment, inflation, and driving habits. They are one layer of safety information rather than a complete assessment of all handling characteristics.
Applying tire markings to purchase and maintenance decisions
When buying new tires, verify that the size, load index, and speed symbol match the vehicle’s door placard or owner manual. Confirm the build date to avoid stocking old inventory, and prefer tires with recent date codes when options are similar. For rotations, replacements, or repairs, record the DOT and build date in service records so age can be tracked accurately. Consider regional climate and typical mileage when setting replacement intervals, and err toward replacing tires showing cracking or age-related wear even if tread depth remains sufficient.
When to replace and when to retain
Legal tread depth limits often mask age-related issues, so rely on both measures. Replace tires when tread reaches the wear indicators, when age approaches or exceeds manufacturer guidance, or when cracking, bulges, or impact damage appear. Retain tires only when they are within recommended age ranges, structurally sound, and properly maintained. Regular pressure checks, alignment inspections, and rotations complement age-based decisions and extend safe service life.