Abstract: Apparatus for changing bicycle gear ratios comprising pedals (1) to the shaft (3) of which a group of coaxial overdrive gears is fixed, with at least an overdrive gear (20) of larger diameter and at least an overdrive gear (2) of smaller diameter; a plurality of sprockets (4) of different diameter, pack-like disposed close to each other and mounted on the shaft (40) of the rear wheel; a roller chain for the transmission of motion from one of the overdrive gears (2; 20) to one of the sprockets (4); a mechanism for shifting the chain (5) from one to another of said sprockets (4), with a cyclist-operated lever-and-wire driving mechanism; and a mechanism for guiding the deviation of the chain (5) from one to the other of the overdrive gears (2; 20) in cooperation with the mechanism for shifting the chain (5) over the sprockets (4) of the rear wheel. The teeth of each overdrive gear exhibiting, in correspondence of a respective sector (M, N), reduced height and thickness with respect to the other teeth.