How to calculate the transmission ratio of the pulley

The pulley is divided into a driving pulley and a driven pulley. The driving pulley is generally installed on the motor shaft, and the driven pulley is installed on the equipment shaft. The pitch diameter (diameter) of the driving pulley at the motor end is smaller and the speed is higher, while the pitch diameter (diameter) of the driven pulley at the equipment end is larger and the speed is relatively slow. Calculated by pitch diameter, assuming that the speed of the driving wheel is n1, the pitch diameter is D1, the speed of the driven wheel is n2, the pitch diameter is D2, and the transmission ratio is X, then X = D1/D2 = N2/N1