10 the wheel 26 is angularly coupled via an inverting idler gear 26a with a second idle toothed wheel 30 on the input shaft 10 and engages simultaneously with a third idle toothed wheel 31 on the input shaft 10 the wheel 27 and the wheel 28 respectively engage with a toothed wheel 32 mounted idly on the driven shaft 12 in a position immediately adjacent to the end 18 of the input shaft 10 and with a toothed wheel 33 mounted idly on the driven shaft 12 in a position adjacent to the bearing 16 the wheels 29 and 30 may be selectively engaged on the shaft 10 in a known manner which is not therefore described in detail by a synchronised coupling 35 interposed between these wheels 29 30 and provided with a sliding control sleeve 36 in a totally analogous way the wheels 31 and 32 can be selectively engaged on the input shaft 10 by a synchronised coupling 37 interposed between these wheels 31 32 and provided with a sliding control sleeve 38 lastly the wheels 32 and 33 can be selectively engaged on the driven shaft 12 by a synchronised coupling 39 interposed between these wheels 32 33 and provided with a sliding control sleeve 40 the sleeves 36 38 and 40 are shown in their intermediate neutral position and can move between a position displaced to the left with reference to the drawing in which they are adapted respectively to engage the wheel 29 on the input shaft 10 the wheel 31 on the input shaft 10 and the.