245/45R18 means a 245 mm section width, a 45% aspect ratio, radial construction and a 18-inch wheel. That works out to a 110.3 mm sidewall and an overall diameter of about 677.7 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.
Common on large saloons — Grandeur, K8, E-Class.
What each number means
| 245 | Section width 245 mm |
|---|---|
| 45 | Aspect ratio 110.3 mm (45% × 245 mm) |
| R | Construction Radial |
| 18 | Wheel diameter 457.2 mm |
| Overall diameter | 677.7 mm 26.7" |
Compatible alternatives
| Size | Overall diameter | Diameter difference |
|---|---|---|
| 235/50R18 | 692.2 mm | +2.1% |
| 245/50R18 | 702.2 mm | +3.6% |
| 255/45R18 | 686.7 mm | +1.3% |
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.