ISO metric tyre sizes
The cryptic code on a tyre sidewall, such as 225/45R17 94W, fully describes its size, construction and ratings under the ISO metric system. Reading it correctly matters for fitment, safety and speedometer accuracy. This tool decodes any standard code into its parts and computes the sidewall height and overall diameter.
Breaking down the code
For the example 225/45R17 94W:
| Field | Value | Meaning |
|---|---|---|
| Section width | 225 | Tread width in millimetres |
| Aspect ratio | 45 | Sidewall height = 45% of width |
| Construction | R | Radial (virtually universal today) |
| Rim diameter | 17 | Wheel rim in inches |
| Load index | 94 | Maximum load per tyre |
| Speed rating | W | Maximum continuous speed |
The sidewall height is 225 × 0.45 = 101.25 mm. The overall diameter is 17 in × 25.4 mm/in + 2 × 101.25 = 431.8 + 202.5 ≈ 634 mm.
How it works
The code parses as width / aspect R rim load-index speed-rating. The section width is in millimetres; the aspect ratio is the sidewall height as a percentage of that width, so sidewall = width × aspect ÷ 100. The letter (usually R for radial) is the construction. The rim diameter is in inches, converted at 25.4 mm/in. Overall diameter = rim(mm) + 2 × sidewall. The two-digit load index and trailing letter map through standard tables to a maximum load in kilograms and a top speed in km/h.
Load index and speed rating: why they matter
These two figures are safety minimums, not suggestions. A vehicle’s door placard, owner’s manual, and the original equipment specification all define a minimum load index and speed rating. Fitting a tyre that falls below either figure is illegal on public roads in most countries and voids insurance.
The load index is an indirect number: for example, 94 maps to 670 kg per tyre (four tyres support a combined 2,680 kg). The speed rating letter maps to a maximum continuous speed: H = 210 km/h, V = 240 km/h, W = 270 km/h, Y = 300 km/h.
You can fit a higher-rated tyre without any concern (for example, W instead of H). You must never fit a lower-rated one.
Changing tyre size: the 3% rule
When fitting an alternative size, keep the overall diameter within roughly 3% of the original specification:
- More than 3% larger: speedometer reads low, ABS and traction-control systems may misjudge wheel speed, and the tyre may foul the wheel arch.
- More than 3% smaller: speedometer reads high, odometer under-counts, and suspension geometry is affected.
A lower aspect ratio (for example 40 instead of 55 on the same width and rim) means a shorter sidewall: sharper steering response and less flex, but more road noise and harshness over poor surfaces. A wider section width increases footprint area but may require a wider rim. Always cross-check fitment against your vehicle’s approved tyre guide or a specialist before purchasing.