A gearbox includes two coaxial primary shafts carrying fixed gearwheels acting as driving gearwheels of the gear trains associated with the forward gears and with the reverse gear, a secondary shaft carrying idle gearwheels acting as driven or intermediate gearwheels of the gear trains associated with the forward gears and with the reverse gear and a lay shaft, and a lay shaft carrying a fixed gearwheel acting as parking gearwheel. The lay shaft is permanently cinematically connected to the secondary shaft via a gear train comprising a first fixed gearwheel carried by the lay shaft, a second idle gearwheel coaxial with the two primary shafts and a third fixed gearwheel carried by the secondary shaft, the first and third fixed gearwheels meshing permanently with the second idle gearwheel.