225/45R17 means a 225 mm section width, a 45% aspect ratio, radial construction and a 17-inch wheel. That works out to a 101.3 mm sidewall and an overall diameter of about 634.3 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.
One of the most common sizes on mid-size saloons — Sonata, K5, 3 Series and similar.
What each number means
| 225 | Section width 225 mm |
|---|---|
| 45 | Aspect ratio 101.3 mm (45% × 225 mm) |
| R | Construction Radial |
| 17 | Wheel diameter 431.8 mm |
| Overall diameter | 634.3 mm 25" |
Compatible alternatives
| Size | Overall diameter | Diameter difference |
|---|---|---|
| 215/50R17 | 646.8 mm | +2% |
| 235/45R17 | 643.3 mm | +1.4% |
| 225/50R17 | 656.8 mm | +3.5% |
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.