In this study, the main concern is estimating the effect of main design parameters on the optimal gear ratio (u2) of a helical worm gearbox. To solve that, a simulation experiment was designed and implemented by a cost optimization program. From the experimental results, the influence of the main design parameters and their interactions on the response u2 was evaluated. In addition, a useful regression model to calculate the optimum gear ratio was proposed based on the experiment data. It was reported that the regression formula is very fit with experimental data, and its use is easy because it is given in the form of an explicit function.