If the letter here is Z, indicating a maximum speed in excess of 240 km/h (149 mph), then a more specific letter W or Y may appear after the load index (see speed rating, below). An optional letter indicating construction of the fabric carcass of the tire: B: bias belt (where the sidewalls are the same material as the tread, leading to a rigid ride) D: diagonal R: radial if omitted, it is a cross-ply tire 1- or 2-digit number: Diameter in inches of the wheel that the tires are designed to fit. There is the rare exception of metric-diameter tires, such as the use of the 390 size, which in this case would indicate a wheel of 390 mm in diameter.