A constant mesh gearbox having a plurality of individually selectable forward speed ratios, the constant mesh gearbox comprising: a casing defining an array of parallel rotational axes, said axes including an input axis, an output axis and a plurality of intermediate axes; and gear pairs for driving between members rotatable on the input axis, the output axis and the intermediate axes, wherein a first reverse speed ratio is provided by connecting an input shaft rotatable on the input axis with an output member rotatable on the output axis via a first shaft rotatable on one of said intermediate axes and a first idler (57, 60) rotatable on a second shaft rotatable on another of said intermediate axes, said first idler (57, 60) comprising a double gear having side by side gear profiles of different diameter, one of said profiles comprising an input and the other of said profiles comprising an output, characterised in that a second reverse speed ratio is provided by connecting said input shaft with said output member via said first shaft and a second idler (55, 59) rotatable on said second shaft, wherein said second idler (55, 59) is a double gear having side by side gear profiles of different diameter, one of said profiles comprising an input and the other of said profiles comprising an output, the input of said second idler is of larger diameter than the output thereof, the second idler (55, 59) is connected for rotation with the input shaft at all times, the second idler (55, 59) drives a gear wheel connectable on demand to said first shaft, and wherein said first and second idlers are alternately connectable to said first shaft via a double acting synchromesh coupler. a casing defining an array of parallel rotational axes, said axes including an input axis, an output axis and a plurality of intermediate axes; and gear pairs for driving between members rotatable on the input axis, the output axis and the intermediate axes, wherein a first reverse speed ratio is provided by connecting an input shaft rotatable on the input axis with an output member rotatable on the output axis via a first shaft rotatable on one of said intermediate axes and a first idler (57, 60) rotatable on a second shaft rotatable on another of said intermediate axes, said first idler (57, 60) comprising a double gear having side by side gear profiles of different diameter, one of said profiles comprising an input and the other of said profiles comprising an output, characterised in that a second reverse speed ratio is provided by connecting said input shaft with said output member via said first shaft and a second idler (55, 59) rotatable on said second shaft, wherein said second idler (55, 59) is a double gear having side by side gear profiles of different diameter, one of said profiles comprising an input and the other of said profiles comprising an output, the input of said second idler is of larger diameter than the output thereof, the second idler (55, 59) is connected for rotation with the input shaft at all times, the second idler (55, 59) drives a gear wheel connectable on demand to said first shaft, and wherein said first and second idlers are alternately connectable to said first shaft via a double acting synchromesh coupler.