An automatic control system for a multiple ratio power transmission mechanism having a hydrokinetic torque converter and multiple ratio gearing, the speed ration of which is controlled by selectively engageable clutches and brakes, the control system including a timing valve circuit that is effective to establish optimum timing between the clutch and brake engagement and release pattern during ratio changes at various driving torques and at various magnitudes of transmission throttle valve pressures, the latter being an indicator of the operator's demand for driving torque.