It is the ratio between the velocities of the driver and the follower or driven. Let d₁ = Diameter of the driver, d₂ = Diameter of the follower, N₁ = Speed of the driver in r.p.m N₂ = Speed of the follower in r.p.m therefore, Length of the belt that passes over the driver, in one minute = π d₁.N₁ 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 Therefore, Velocity ratio, N₂/N₁ = d₁/d₂.