215/55R18 means a 215 mm section width, a 55% aspect ratio, radial construction and a 18-inch wheel. That works out to a 118.3 mm sidewall and an overall diameter of about 693.7 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.
Used on some trims of EVs including the Ioniq 5 and EV6.
What each number means
| 215 | Section width 215 mm |
|---|---|
| 55 | Aspect ratio 118.3 mm (55% × 215 mm) |
| R | Construction Radial |
| 18 | Wheel diameter 457.2 mm |
| Overall diameter | 693.7 mm 27.3" |
Compatible alternatives
| Size | Overall diameter | Diameter difference |
|---|---|---|
| 235/55R19 | 741.1 mm | +6.8% |
| 215/60R17 | 689.8 mm | -0.6% |
| 225/55R18 | 704.7 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.