Every tyre carries a wealth of information moulded into its sidewall. To the uninitiated, these numbers and letters appear cryptic—a jumble of codes that seem designed to confuse rather than inform. In reality, they're a comprehensive specification sheet that tells you everything you need to know about your tyre's capabilities and appropriate use. Learning to read these markings empowers you to make informed decisions about replacement tyres and understand your current rubber's characteristics.
The Size Designation
The most prominent marking on any tyre sidewall is the size designation. A typical example reads: 225/45R17 91W. Each element conveys specific information:
225 — Section Width
This number indicates the tyre's width in millimetres, measured from sidewall to sidewall when mounted on the recommended rim and inflated to the correct pressure. In our example, the tyre is 225mm (approximately 8.9 inches) wide.
45 — Aspect Ratio
The aspect ratio expresses the sidewall height as a percentage of the section width. A 45 aspect ratio means the sidewall height is 45% of 225mm, equalling approximately 101mm. Lower numbers indicate a shorter sidewall (low-profile appearance), while higher numbers indicate taller sidewalls (more comfortable ride, greater protection against pothole damage).
R — Construction Type
The letter R indicates radial construction, where the tyre's internal plies radiate from the centre at 90 degrees to the direction of travel. Virtually all modern passenger vehicle tyres are radial. You may occasionally encounter D (diagonal/bias-ply) on older vehicles, trailers, or speciality applications.
17 — Rim Diameter
This number indicates the wheel diameter in inches that the tyre is designed to fit. In our example, the tyre fits a 17-inch wheel. Using a tyre on the wrong rim size is dangerous and impossible to mount correctly.
Your vehicle's correct tyre size is listed on the placard inside the driver's door jamb, in the owner's manual, or on the fuel filler door. Never assume your current tyres are correct—previous owners may have fitted non-standard sizes.
Load Index and Speed Rating
Following the size designation, you'll find the service description—in our example, 91W.
Load Index (91)
The load index is a numerical code corresponding to the maximum weight each tyre can safely carry at its maximum rated speed. The number relates to a standardised table:
- 85 = 515 kg per tyre
- 87 = 545 kg per tyre
- 91 = 615 kg per tyre
- 95 = 690 kg per tyre
- 100 = 800 kg per tyre
- 105 = 925 kg per tyre
Always match or exceed the load index specified by your vehicle manufacturer. Under-rated tyres risk failure under load.
Speed Rating (W)
The speed rating indicates the maximum speed at which the tyre can safely carry its rated load. Common ratings include:
- T = 190 km/h
- H = 210 km/h
- V = 240 km/h
- W = 270 km/h
- Y = 300 km/h
While you'll never legally drive at these speeds in Australia, the rating also relates to heat handling capability. Higher-rated tyres manage heat better during sustained high-speed driving.
- Load Index: Must meet or exceed vehicle specification
- Speed Rating: Should meet or exceed vehicle specification
- Higher ratings indicate greater capability but often higher cost
- Never fit lower-rated tyres than originally specified
DOT Code and Manufacturing Date
Every tyre sold in Australia carries a DOT (Department of Transportation) code, indicating compliance with safety standards. More importantly for consumers, the last four digits of this code reveal the manufacturing date.
Look for markings beginning with "DOT" followed by a series of letters and numbers. The final four digits are the date code: 2519 means the tyre was manufactured in the 25th week of 2019.
This date matters because rubber degrades with age. Most manufacturers recommend replacement after six years of service, and tyres should never be used after ten years regardless of appearance. When buying new tyres, check the DOT code to ensure you're not receiving old stock—tyres should be manufactured within the last year or two.
The DOT code appears on only one sidewall. If you can't find it, check the other side of the tyre. For tyres already mounted, this may require viewing from underneath the vehicle or removing the wheel.
UTQG Ratings
Many tyres display UTQG (Uniform Tire Quality Grading) ratings—a system developed in the United States but commonly shown on tyres sold in Australia.
Treadwear
A comparative number indicating relative tread life. A tyre rated 400 should last twice as long as one rated 200 under identical conditions. Note that these are manufacturer-reported and tested on specific surfaces—use them for comparison within brands rather than absolute predictions.
Traction
Rated AA, A, B, or C, indicating the tyre's ability to stop on wet concrete and asphalt. AA offers the best wet grip, while C represents minimum acceptable performance. Most quality tyres rate A or AA.
Temperature
Rated A, B, or C, indicating resistance to heat generation at sustained speed. A-rated tyres handle heat best, important for highway driving and hot Australian conditions.
Additional Markings
M+S (Mud and Snow)
Common on all-season and all-terrain tyres, this marking indicates the tread pattern is designed to provide traction in mud and light snow. It's a self-declared designation with no standardised testing requirement.
Three-Peak Mountain Snowflake (3PMSF)
A more rigorous winter capability certification. Tyres bearing this symbol have passed standardised traction tests in snow conditions. Relevant for Australian alpine travel and required in some overseas conditions.
Rotation Arrows
Directional tyres feature arrows on the sidewall indicating the intended rotation direction. These tyres must be mounted with the arrow pointing in the direction of forward travel. Mounting backwards significantly impairs performance, particularly in wet conditions.
Inside/Outside
Asymmetric tyres have different tread patterns on inner and outer sections. Sidewall markings indicate which side faces outward when mounted. Incorrect mounting reduces performance and may accelerate wear.
Maximum Pressure
You'll find a maximum inflation pressure marked on the sidewall, typically stating something like "Max Press 350 kPa Cold". This is the maximum safe pressure for the tyre—not the recommended operating pressure. Always use the pressure specified by your vehicle manufacturer, which is typically lower.
Reinforced (XL) or Extra Load
Some tyres carry Reinforced or Extra Load (XL) designations, indicating they're built to handle higher loads than standard tyres of the same size. These are commonly specified for heavier vehicles and should be fitted when the manufacturer requires them.
Using Sidewall Information
Understanding sidewall markings helps you:
- Choose correct replacements: Match size, load index, and speed rating to vehicle specifications
- Verify new tyre freshness: Check DOT date codes when purchasing
- Assess tyre age: Know when replacement is due based on age, not just tread
- Compare tyres: Use UTQG ratings to understand relative characteristics
- Ensure correct fitment: Observe rotation direction and inside/outside markings
The next time you inspect your tyres, take a moment to read their story. That sidewall carries more information than most drivers realise—information that helps you make better decisions about the safety-critical components connecting your vehicle to the road.