A gearbox with reduced encumbrance, comprising a primary shaft carrying primary gearings (30, 31, 32, 33, 34, 35), a secondary shaft parallel to said at least one primary shaft which carries secondary gearings (37, 38, 39) of odd-number gears and secondary gearings (40, 41, 42) of even-number gears which can be activated in engagement with the respective primary gearings (30, 31, 32, 33, 34, 35) of said primary shaft by means of a cam shaft coaxial with said secondary shaft, carrying cams (54, 55, 56, 57, 58, 59), spaced axially and angularly offset in a helical fashion with respect to each other by an angle α, so as to be brought one at a time in correspondence with said secondary gearings (37, 38, 39, 40, 41, 42) positioned on said secondary shaft which follows the rotation of said cam shaft with respect to said secondary shaft, rotation means (29, 50) of said cam shaft being envisaged with respect to said secondary shaft, and means for the selective engagement (51, 52, 53, 49) between said primary gearings (30, 31, 32, 33, 34, 35) and said secondary gearings (37, 38, 39, 40, 41, 42), due to the rotation of said cam shaft with respect to said secondary shaft, wherein said rotation is driven by means of an actuator, wherein said selective engagement means between said primary gearings (30, 31, 32, 33, 34, 35) and said secondary gearings (37, 38, 39, 40, 41, 42) consist of barrels slidingly positioned in openings positioned angularly offset in a helical fashion with respect to each other in hollow side walls of said secondary shaft in a complementary position with respect to that of said cams (54, 55, 56, 57, 58, 59) of said cam shaft, by which cams (54, 55, 56, 57, 58, 59) they are actuated to radially slide, each secondary gearing (37, 38, 39, 40, 41, 42) comprising at least one seat for receiving a respective barrel when raised, wherein: - a second primary shaft is envisaged, which is coaxial to said first primary shaft and carries primary gearings (30, 31, 32) of odd-number gears, whereas said first primary shaft carries primary gearings (33, 34, 35) of even-number gears, each of said primary shafts (11, 12) integrally carrying a respective clutch (13, 14) which is coaxial and can be independently actuated, - said cams (54, 55, 56, 57, 58, 59) are in a number of at least two for each secondary gearing (37, 38, 39, 40, 41, 42) which, in turn, comprises at least two seats for receiving respective barrels when raised, said cams, seats and barrels being positioned on parallel planes and in correspondence with the centre-line of the secondary gears (37, 38, 39, 40, 41, 42) of the odd-number and even-number gears. - a second primary shaft is envisaged, which is coaxial to said first primary shaft and carries primary gearings (30, 31, 32) of odd-number gears, whereas said first primary shaft carries primary gearings (33, 34, 35) of even-number gears, each of said primary shafts (11, 12) integrally carrying a respective clutch (13, 14) which is coaxial and can be independently actuated, - said cams (54, 55, 56, 57, 58, 59) are in a number of at least two for each secondary gearing (37, 38, 39, 40, 41, 42) which, in turn, comprises at least two seats for receiving respective barrels when raised, said cams, seats and barrels being positioned on parallel planes and in correspondence with the centre-line of the secondary gears (37, 38, 39, 40, 41, 42) of the odd-number and even-number gears.