Abstract—In this paper, we present a study on the optimum determination of partial transmission ratios of a mechanically driven system using a V-belt and a helical gearbox with second-step double gear sets in order to get the minimum size of the system. The chosen objective function was the cross-sectional dimension of the system. In the optimization problem, the design equation for the pitting resistance of a gear set was investigated. Besides, the equations of the moment equilibrium condition of a mechanical system including a V-belt and a helical gearbox with second-step double gear sets and their regular resistance condition were analyzed. Based on the results of this study, effective formulas for the calculation of the partial ratios of the V-belt and helical gearbox with second-step double gear sets were proposed. Using explicit models, the partial ratios can be easily and accurately determined.