To solve the problem wherein a fluid three-element torque converter has frequent fluid slip to cause poor fuel economy, and weak medium speed torque, in particular, to involve a gear transmission group and a hydraulic automatic control circuit connected in series to each other into a complicated structure to improve them, and has inferior reliability because of less sufficient durability in using a belt, a sliding top and electromagnetic control, than fluid-type one.

As shown in Fig.1, this fluid parallel auxiliary four-speed automatic transmission comprises a reduction gear transmission and a direct-coupled clutch connected in parallel to the fluid three-element torque converter to form such a double safety structure that it is used as the fluid three-element torque converter during travel even when a trouble occurs in the direct-coupled clutch or the reduction gear. A torque converter stator is rotated to add fluid reaction torque in parallel to the gear transmission for reinforcement while eliminating the need for a first-speed reduction gear.