A speed control mechanism for controlling selective engagement of the self- propelled transmission assembly for a walk-behind lawn mower is provided. The speed control mechanism is also configured to control the relative output speed of the transmission assembly. The speed control mechanism includes a speed engagement assembly (33)and a speed adjustment assembly (32), wherein the speed engagement assembly (33) includes a pair of rotatable levers (34,36) for causing the transmission assembly to actuate between a disengaged position and an engaged position and the speed adjustment assembly (32) includes a rotatable knob (42) for causing the relative output speed of the transmission assembly to the wheels of the lawn mower to increase or decrease.
