Assembly (57) for a speed reducer of a rotary electric machine, comprising: a ring gear (46) secured to a transverse wall (58); a base plate (60) to be securely and rotatably mounted relative to a mounting (16) of the machine; an axially acting resilient means, such as an axially acting resilient disc (64); and an added hub (68) on which the transverse wall (58) of the ring gear (46), the base plate (60), and the resilient means (64) are mounted. The hub (68) has at least one first shoulder (71) for supporting the axially acting resilient means (64), and at least one second shoulder (72) for supporting the transverse wall (58) of the ring gear (46). The assembly also relates to a starter, particularly for a motor vehicle.