225/55R17 means a 225 mm section width, a 55% aspect ratio, radial construction and a 17-inch wheel. That works out to a 123.8 mm sidewall and an overall diameter of about 679.3 mm.
The aspect ratio is a proportion, not a measurement: the same 45 gives a different sidewall height on a different width, and that changes both ride and handling. A lower ratio holds a corner better and transmits more of the road surface.
Comfort trims of mid-size saloons — SM6, Malibu, Passat.
What each number means
| 225 | Section width 225 mm |
|---|---|
| 55 | Aspect ratio 123.8 mm (55% × 225 mm) |
| R | Construction Radial |
| 17 | Wheel diameter 431.8 mm |
| Overall diameter | 679.3 mm 26.7" |
Compatible alternatives
| Size | Overall diameter | Diameter difference |
|---|---|---|
| 215/55R17 | 668.3 mm | -1.6% |
| 225/50R17 | 656.8 mm | -3.3% |
| 235/55R17 | 690.3 mm | +1.6% |
Load index and speed rating
After the size itself, a tyre wall carries something like 91V. The number is the load index — the weight one tyre can carry, 91 meaning 615 kg — and the letter is the speed rating, V meaning 240 km/h.
Never drop below the load index the manufacturer specified, even if the size matches. The original specification is printed on the label inside the driver’s door.