Motor vehicle gearbox

motor vehicle gearbox, comprising at least two concentrically arranged alternately driveable first and second input shafts (6,9), first and second layshafts (12,15), primary gears carried by the first input shaft (6) and the first layshaft (12), said primary gears (10,11) engaging each other for transmitting torque from the first input shaft (6) to the first layshaft (12), primary gears (13,14) carried by the second input shaft (9) and the second layshaft (15), said primary gears engaging each other for transmitting torque from the second input shaft (9) to the second layshaft (15), an output shaft (20), gears (16,17,18,19,21,22) carried by the layshafts and the output shaft, said gears engaged in each other in pairs for transmitting torque from either layshaft to the output shaft and of which at least one gear in each pair is a releasable idler gear, and synchronizing means (30) comprising firstly a pair of synchronizing gears of different diameters rotatably mounted on one of the layshafts, said gears being in driving engagement with gear rings (13,39) on one input shaft (9), and secondly friction means (34), by means of which a driving connection between one synchronizing gear (32,333) and the layshaft (12) can be established to adapt the rotational speed of the layshaft which is driveably coupled to the momentarily undriven input shaft, to a rotational speed determined by the selected gear speed, at least the synchronizing gear (33) of smaller diameter being coordinated with engaging means (60) which permits transmission of higher torque to its layshaft than what is required for said rotational speed adaption, characaterized in that the primary gear (14) of the second layshaft (15) is coordinated with engaging means (55), whereby the primary gear (14) is releasable from and lockable to its shaft (14), and that the primary gear is releasable from its shaft at the same time as the synchronizing gear (33) of smaller diameter is lockable, by means of its engagement means (60) to its shaft (12) to permit torque transmission from the second input shaft (9) via said synchronization gear (33) and inter-engaging gears (11,12) on the first and second layshafts (12,15) to the output shaft (20).