In this paper, the design criteria for high efficiency planocentric transmissions is presented. The space solution of a single stage planocentric transmission is limited by geometric constraints. These constraints are produced basically by the interference between pinion teeth and gear teeth. Special interest is focused on the interferences of internal gears and a detailed analysis of the influence of teeth parameters, such as addendum coefficient and pressure angle, is included. Conclusions are drawn from the analysis of the interferences and a design criteria is outlined. It was found that the space solution can be wider if pinion and gear are designed with stub teeth and high pressure angle.