Rectangular seals - a special type of face seals - are currently used in rotating connections of automatic automotive transmissions (double clutch transmissions, continuously variable transmissions, and 6-automated transmission) for the oil pressure supply of rotating control and actuator elements. Friction losses of several kilowatts can occur in the sealing system, causing a considerable reduction in efficiency. As a result of ongoing progress in power train design, sliding speeds and fluid pressures continue to increase. Therefore, due to friction related temperature rise in the sealing contact, extensive oil carbonization may occur. For both the reasons, developing friction optimized seals is crucial. For cost and design reasons grey cast iron rings are more and more substituted by modern, high temperature resistant thermoplastics. These materials also exhibit better friction and wear performance which can further be improved by adding polytetrafluorethylen or graphite. Because of their lower stiffness compared with metallic components, special thoughts have to be given to the design of thermoplastic rectangular seals. This paper describes how an improved geometry with optimized friction and wear performance was achieved by combining experimental investigations and numerical calculations on thermomechanical deflections. © IMechE 2007.