A motor-vehicle gearbox comprising a plurality of gearings that define corresponding forward gear ratios (I, II) and a reverse gear ratio, each gearing comprising a first gear wheel carried by a primary shaft (PS) of the gearbox and a second gear wheel , coupled to said first gear wheel, carried by a secondary shaft (SS) of the gearbox, the primary shaft (PS) being rotatable about a first axis of rotation (XPS), and the secondary shaft (SS) being rotatable about a second axis of rotation (XSS), parallel to the first axis of rotation (XPS), wherein in at least one gearing (I) of said plurality of gearings: the first gear wheel is permanently connected in rotation to the primary shaft (PS); and the second gear wheel is mounted freely rotatable with respect to the secondary shaft (SS), said second gear wheel being connected in rotation to an epicyclic unit coaxial to the secondary shaft (SS), said epicyclic unit being moreover operatively connected to a range-selector device , said epicyclic unit comprising: - a sun gear , which is coaxial to the second gear wheel of the gearing (I); - a ring gear , which is coaxial to the second gear wheel of the gearing (I) and is connected in rotation thereto; and - a planetary unit , which is coaxial to the second gear wheel of the gearing (I) and includes one or more planetary gears carried by a planet carrier ; wherein the range-selector device includes a first operating condition (22+) in which said sun gear is connected in rotation to said ring gear , and a second operating condition (22-) in which rotation of said sun gear is blocked, and wherein the gearing (I) is moreover operatively associated to an engagement device (S1/2) configured for connecting selectively in rotation said planet carrier to the secondary shaft (SS). the first gear wheel is permanently connected in rotation to the primary shaft (PS); and the second gear wheel is mounted freely rotatable with respect to the secondary shaft (SS), said second gear wheel being connected in rotation to an epicyclic unit coaxial to the secondary shaft (SS), said epicyclic unit being moreover operatively connected to a range-selector device , said epicyclic unit comprising: - a sun gear , which is coaxial to the second gear wheel of the gearing (I); - a ring gear , which is coaxial to the second gear wheel of the gearing (I) and is connected in rotation thereto; and - a planetary unit , which is coaxial to the second gear wheel of the gearing (I) and includes one or more planetary gears carried by a planet carrier ; - a sun gear , which is coaxial to the second gear wheel of the gearing (I); - a ring gear , which is coaxial to the second gear wheel of the gearing (I) and is connected in rotation thereto; and - a planetary unit , which is coaxial to the second gear wheel of the gearing (I) and includes one or more planetary gears carried by a planet carrier ; wherein the range-selector device includes a first operating condition (22+) in which said sun gear is connected in rotation to said ring gear , and a second operating condition (22-) in which rotation of said sun gear is blocked, and wherein the gearing (I) is moreover operatively associated to an engagement device (S1/2) configured for connecting selectively in rotation said planet carrier to the secondary shaft (SS). <EOS>