In a gear box design, it is quite difficult for a designer to quickly find an optimum solution, while the space restriction is given and the weight is to be minimized. Also the manufacturing cost, total power loss and other relevant factors have to be considered. Solving such a problem is very time-consuming, if different variants of reducers have to be evaluated carefully. A new optimization tool, based on KISSsys software, permits to layout automatically a complete gearbox using functions for the layout of gear stages, shaft dimensions and bearings, if the required torque capacity, life time, safety factors and the total ratio are given. This tool is used to generate a complete set of different variants of gear reducers. The main results are displayed in 3D graphics, showing weight, costs and efficiency of the different variants. Another graph shows the casing dimensions of the variants in three dimensions in order to check the compliance with space restriction. The use of this tool in two typical engineering applications gives very good results; the potential for weight reduction is higher than expected, going up to 30%. Manufacturing price reduction is even higher, going up to 50%.