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The gearbox gives a selection of gears for different driving conditions: standing start, or cruising on level surfaces, climbing a hill,. The lower the gear, the slower the road wheels turn in relation to the engine speed.
Modern cars with manual transmissions have four or five forward speeds and one reverse, along with a neutral position.
The gear lever, operated by the driver, is connected to a series of selector rods in the top or side of the gearbox. The selector rods lie parallel with shafts carrying the gears.
The most prominent design is the constant-mesh gearbox. It has three shafts: the input shaft, the lay shaft and the main shaft, which run in bearings in the gearbox casing.
There is also a shaft on which the reverse-gear idler pinion rotates.
The engine drives the input shaft, which drives the layshaft. The layshaft rotates the gears on the mainshaft, but these rotate freely until they are locked by means of the synchromesh device, which is splined to the shaft.
It is the synchromesh device which is generally operated by the driver, with a selector rod with a fork on it which moves the synchromesh to engage the gear.