Adjustable gearbox for vehicles, able to maintain constant ground clearance

The adjustable gearbox (8) for vehicles with constant clearance from the ground (4) according to the invention is provided for the driving wheel or wheels (5, 6) of a vehicle, the box (8) comprises a reducing unit (8a) with three axes (11a, 13a, 33'; 11a, 13a, 33"), the axis (33', 33") of the driving wheels (5, 6) corresponds to the third axis of the reducing unit and can be positioned in a plurality of points along a path (L), substantially in the form of an arc of a circle and positioned on a plane perpendicular to the ground (4).