The manual gearbox takes the engine rotational torque and passes it through varying arrangements of gears (cogs) that convert it to a different output speed that turns the driven axle(s). Without gearboxes cars would be practically useless and the output of the combustion engine would always directly correspond to the speed of the wheels.
Most manual gearboxes have 5 or 6 forward gears and one reverse gear. The gear ratios are different depending on the type of car and the engine characteristics, performance cars can have longer gearing so that cars can get to 80+mph in second gear, whereas small town cars will have much shorter ratios.
Manual gearboxes take a lot of stress and quite often drivers with poor gear changing technique can put extra strains on the components of the gearbox. Poorly maintained gearboxes can suffer from worn out components and seize up, so its important that a gearbox is maintained correctly and the oil level is maintained.
Replacement manual gearboxes can cost thousands when bought from the manufacturer. Generally you won't buy a brand new gearbox for an older car that is out of warranty, it makes a lot more sense to buy a refurbished unit, these an cost from £400 for a car like a Ford Focus.
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