Transmission gears can be classified into four groups—input gear, countershaft gears, main shaft gears, and the reverse idler gear. The input gear turns the countershaft gears, the countershaft gears turns the main shaft gears, and, when engaged, the reverse idler gear.
In low gear, a small gear on the countershaft drives a larger gear on the main shaft, providing for a high gear ratio for accelerating. Then, in high gear, a larger countershaft gear turns a small main shaft gear or a gear of equal size, resulting in a low gear ratio, allowing the vehicle to move faster. When reverse is engaged, power flows from the countershaft gear, to the reverse idler gear, and to the engaged main shaft gear. This action reverses main shaft rotation.
