The invention concerns a driving device for a machine comprising a crank gear (3) actuated by a user's force, operating on the pedals (4) arranged at the free ends of two pedal cranks (5) integral with said crank gear (3), the latter being connected to a driving pinion (6) of a main wheel by means of a free intermediate gear wheel (8) comprising at least two toothed stages (8a, 8b) one of which (8b) is connected to the crank gear (3) and the other (8a) to a driving pinion (6) by any connecting means. The invention is characterised in that the intermediate gear wheel (8) includes at least one first toothed stage (8b) connected to the standard crank gear (3) and the diameter of which is not greater than that of said crank gear (3) and a second toothed stage (8a) connected to the driving pinion (6) of the wheel (7) and the diameter of which is greater than that of said crank gear (3), so as to obtain a velocity gain of the machine for a given torque supplied by the user.