The Main Size Code
A typical tyre size looks like this: 205/55R16 91V. Once you know what each part means, reading it becomes as easy as reading a shoe size.
Width (205)
The tyre's width in millimetres, measured from sidewall to sidewall.
Aspect ratio (55)
Sidewall height as a percentage of the width.
Construction & rim size (R16)
"R" means radial construction. "16" is the wheel rim diameter in inches.
Load index (91)
A code showing the maximum weight the tyre can safely carry.
Speed rating (V)
A letter showing the maximum safe speed for the tyre.
Understanding Load Index
The load index is a numerical code that corresponds to a specific maximum weight per tyre, found on standardised load index charts. Fitting tyres with a load index lower than your vehicle manufacturer recommends can be unsafe, particularly when the vehicle is fully loaded with passengers and cargo.
Understanding Speed Ratings
Speed rating letters, ranging from L up to Y, indicate the maximum speed a tyre can safely sustain. It is important to fit tyres that meet or exceed your vehicle manufacturer's recommended speed rating, since a lower-rated tyre may not be built to handle the heat and stress generated at higher speeds.
The DOT Code and Manufacturing Date
Near the size code, you will find a DOT code ending in four digits, such as 3223, indicating the 32nd week of 2023. This is one of the most overlooked but important markings, since rubber degrades over time even if a tyre has never been fitted.
Tread Wear Indicators
Look closely into the main grooves of the tread and you will find small raised bars running across them. Once the tread wears down to the same level as these bars, the tyre has reached the legal minimum and needs replacing.
Other Markings Worth Knowing
| Marking | What it tells you |
|---|---|
| M+S (Mud and Snow) | Extra grip rating for loose or slippery surfaces |
| Directional arrow | Correct rotation direction for the tread pattern |
| Max pressure (kPa/PSI) | Maximum cold inflation pressure, not your recommended running pressure |
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FAQs
What does the DOT code actually tell me?
The last four digits show the week and year of manufacture, helping you check how old a tyre is before buying it.
Can I fit a tyre with a lower load index than recommended?
No, this can be unsafe, particularly when the vehicle is fully loaded with passengers and cargo.
What happens once tread reaches the wear indicator bars?
The tyre has reached the legal minimum tread depth and needs to be replaced.