Full definition
The ratio between driver (input) and driven (output) pulley speeds, determining speed reduction or multiplication in a belt drive system. Calculated as i = n1/n2 = D2/D1, where n = RPM and D = pitch diameter. A ratio of 3:1 means the driven shaft rotates at one-third the driver speed while receiving approximately three times the torque (minus efficiency losses). Maximum practical ratio per stage: 6:1-8:1 for V-belts (limited by minimum wrap angle on small pulley), 12:1 for timing belts. For higher ratios, use two-stage drives or a speed reducer. Exact ratio calculation must use pitch diameters, not outside diameters. Allowance for belt creep (~1% for V-belts) gives actual driven speed slightly less than theoretical.