The velocity ratio of a belt drive with slip is calculated by considering the reduction in speed at each stage. If $N_1$ is the driver speed and $N_2$ is the follower speed, and $s_1$ and $s_2$ are the percentage slips at the driver and follower respectively, the velocity ratio $\frac{N_2}{N_1} = (1 - s_1/100)(1 - s_2/100)$ for equal diameter pulleys.