A speed change mechanism in which the gear ratio is selected by changing the direction of rotation of the power input includes first and second planetary gear sets enclosed within a hub shell. A DC electric motor is coupled to the sun gears of both gear sets and drives them simultaneously in the same rotary direction. In one of the gear sets the carrier arm is fixed and the ring gear is the output and in the other gear set the ring gear is fixed and the carrier arm is the output. A first one-way clutch transmits drive from an output of the first gear set to the hub shell and second one-way clutch transmits drive from an output of the second gear set to the hub shell, both clutches operating in the same direction. The outputs of the gear sets are driven in opposite rotary directions so that drive is transmitted to the hub shell either through the first gear set or the second gear set depending on the direction of rotation of the power input to provide two different gear ratios.