How do you calculate the gear ratio of a compound gear train?

For a compound gear train, the overall gear ratio is the product of individual stage ratios. If gears are numbered 1,2,3,4... with 1 as driver, Gear Ratio = (Z2/Z1) x (Z4/Z3) x ... where Z is the number of teeth. For a two-stage reduction with 20-tooth driver meshing 60-tooth gear (shaft 1), and 18-tooth gear on same shaft meshing 54-tooth output gear, ratio = (60/20) x (54/18) = 3 x 3 = 9:1 speed reduction.