Abstract: A variable ratio drive mechanism comprises a reciprocable input member having a toothing, an output gear and an intermediate pivoted gear meshing with the toothing of said input member and said output gear to effect a driving connection between the input member and the output gear. An adjustment device is provided for selectably positioning the pivot of said intermediate gear whereby the distance between the pivot of the intermediate gear and the point of meshing engagement of the intermediate gear and the output gear can be infinitely varied. Preferably the pivot can be moved through a mid-position to positions either side of the mid-position.