The hydraulic circuitry of a factory installed automatic automotive transmission is modified to replace an underdrive accumulator spring with a shorter spring and a separate lower rated spring washer operatively associated with the shorter replacement underdrive accumulator spring. The combined replacement spring and spring washer conjointly act on an underdrive accumulator piston to move it in two predetermined distances, in two separate continuous stages, namely, movement of the piston for a first predetermined distance is controlled by the spring washer, while movement of the piston for a second predetermined distance is controlled by the replacement spring. The spring washer has a lower spring rating than the replacement spring, so that less fluid pressure is required to move the piston the total distance traveled, resulting in less fluid pressure applied to an underdrive clutch than in the factory installed transmission. The spring ratings of the replacement spring and the spring washer are selected such that the total fluid pressure required to move the piston both the first and second predetermined distances is less than the fluid pressure required to overcome the resilient force of underdrive piston return springs acting on an underdrive apply piston so that a lower fluid pressure is applied to an underdrive clutch.