Abstract: A fluid coupled speed retarder generating a restraining torque on a vehicule wheel system comprises a restraining shaft (50) coupled by a viscous fluid to a wheel (32) and axle (54) system through a coupling (30) which includes an impeller (48) with radial vanes (77) connected to the restraining shaft and a housing (44) with further radial vanes (46), the housing connected for rotation with the wheel (34). A braking force is developed upon demand by a selective locking of the restraining shaft (50) from motion, the relative motion between the impeller and housing in the viscous coupling generating the barking force.