255/40R19 means a 255 mm section width, a 40% aspect ratio, radial construction and a 19-inch wheel. That works out to a 102 mm sidewall and an overall diameter of about 686.6 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.
A low-profile size used on performance saloons such as the G70 and M-package BMWs.
What each number means
| 255 | Section width 255 mm |
|---|---|
| 40 | Aspect ratio 102 mm (40% × 255 mm) |
| R | Construction Radial |
| 19 | Wheel diameter 482.6 mm |
| Overall diameter | 686.6 mm 27" |
Compatible alternatives
| Size | Overall diameter | Diameter difference |
|---|---|---|
| 245/40R19 | 678.6 mm | -1.2% |
| 255/35R19 | 661.1 mm | -3.7% |
| 265/40R19 | 694.6 mm | +1.2% |
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.