It is the ratio between the velocities of the driver and the follower or driven. It may be expressed, mathematically, as discussed below:

Let d1 = Diameter of the driver, d2 = Diameter of the follower,

N1 = Speed of the driver in r.p.m., and N2 = Speed of the follower in r.p.m.

Length of the belt that passes over the driver, in one minute = π d1N1

Similarly, length of the belt that passes over the follower, in one minute = π d2 N2

Since the length of belt that passes over the driver in one minute is equal to the length of belt that passes over the follower in one minute, therefore

π d1 N1 = π d2 N2

When the thickness of the belt (t) is considered, then velocity ratio,