A bicycle is disclosed with a rhomboidal (preferably 60<o> - 120<o>) drive sprocket gear (10) and a circular driven sprocket on the rear driven wheel (26). This gear arrangement varies the overall power input from the drive sprocket (10) to the driven gear (32). The shape of the drive sprocket (10) and the location of the drive pedal (27) changes the gear ratio so that a low gear ratio is utilized during the horizontal portion of movement of the cyclist's legs and a high gear ratio is utilized during the vertical movement of the legs. The gear ratio with the rhomboidal gear (10) is relatively constant during horizontal movement and abruptly changes at the approach to and during vertical movement of the cyclist's legs. While the rhomboidal gear is shown primarily for bicycles it may be used in other systems for converting reciprocal to rotary movement with a change in gear ratio. <IMAGE>
