How to read a tyre size: what 205/55 R16 91V means
Every tyre carries a marking such as 205/55 R16 91V on its sidewall. It is not arbitrary: each part describes a specific dimension or capability. Knowing it helps you buy the right tyre, compare sizes and understand why your speedometer changes.
The size, part by part
| Part | In the example | Meaning |
|---|---|---|
| Width | 205 | Section width of the tyre, in millimetres. |
| Aspect ratio | 55 | Sidewall height as a percentage of the width: 205 × 0.55 = 112.75 mm. |
| Construction | R | Radial, the structure of virtually every modern car tyre. |
| Rim | 16 | Diameter of the wheel it fits, in inches. |
| Load index | 91 | Maximum load per tyre from a standard table: 91 means 615 kg. |
| Speed rating | V | Maximum speed the tyre is approved for: V means 240 km/h (149 mph). |
Overall diameter: the number that matters most
The first three numbers give you the unloaded tyre diameter:
Diameter = rim × 25.4 + 2 × width × aspect / 100
For a 205/55 R16: a 16 × 25.4 = 406.4 mm rim plus two 112.75 mm sidewalls gives 631.9 mm.
That diameter sets how far the car travels with each wheel turn. Fit a size with a different diameter and both your real speed at a given rpm and your speedometer reading change. You can check it with the tire size calculator, which compares two sizes, shows the speedometer error and suggests equivalent sizes. To see how speed changes in each gear, use the gear ratio calculator.
Common speed ratings
| Rating | Maximum speed |
|---|---|
| Q | 160 km/h (99 mph) |
| R | 170 km/h (106 mph) |
| S | 180 km/h (112 mph) |
| T | 190 km/h (118 mph) |
| H | 210 km/h (130 mph) |
| V | 240 km/h (149 mph) |
| W | 270 km/h (168 mph) |
| Y | 300 km/h (186 mph) |
The speed rating and load index of any tyre you fit should be at least those listed in your vehicle’s documentation.
Manufacturing date
Look for a four-digit group, usually at the end of the DOT code. The first two digits are the week and the last two the year: 2324 means week 23 of 2024. It tells you a tyre’s real age, which matters because rubber ages even when there is tread left.
Other markings you may see
- M+S (mud and snow): the manufacturer declares a tread designed for mud and snow.
- Mountain and snowflake symbol (3PMSF): the tyre has passed a snow performance test.
- XL or Reinforced: reinforced casing with a higher load index than the standard version of the same size.
Before changing size
The sizes approved for your car are listed in its registration documents and the manufacturer’s documentation. A different size can affect type approval, roadworthiness inspections and insurance, so check before you buy.