There are many sound reasons, both economic and mechanical, for using an automatic transmission if it has high efficiency. The G.K.N.-S.R.M. transmission links an epicyclic gear train with an unconventional converter blade system to change conditions of fluid flow and give a very high stall torque ratio. Hydraulic efficiencies in the converter stages are discussed and torque distribution in the system is analysed. Use is made of existing elements to give hydraulic braking at negligible additional cost. Simple modifications to the blade system allow accurate matching of the transmission to the engine while auxiliary gearing may be provided to give a wider ratio spread if required. The transmission control system is described.Another version of the converter with variable pitch impeller blades has been designed for use with turbine engines up to 400 hp which may, with advantage, be of single-shaft type.