The gearbox comprises a pair of coaxial input shafts, that is, an inner input shaft and an outer input shaft, a first, upper output shaft, a second, lower output shaft, and an intermediate shaft. A driven gearwheel for a fourth gear and a driven gearwheel for a reverse gear are idly mounted on the opposite ends of the upper output shaft, the driven gearwheel meshing with an intermediate gearwheel idly mounted on the intermediate shaft. A driven gearwheel for a sixth gear and a driven gearwheel for a first gear are idly mounted on opposite ends of the lower output shaft. A first driving gearwheel is drivingly connected for rotation with the outer input shaft at an end thereof and meshes both with the driven gearwheel for the fourth gear and with the third driven gearwheel for the sixth gear. A second driving gearwheel meshing with the driven gearwheel for the first gear and a third driving gearwheel meshing with the intermediate gearwheel are drivingly connected for rotation with the inner input shaft at the opposite end thereof. The third driving gearwheel, the driven gearwheel for the reverse gear and the intermediate gearwheel are straight gearwheels.