In any vehicle the job of the transmission is to change the speed ratio between the engine and the wheels of the vehicle and to allow the engine to use its limited range of speeds and provide a wide range of output speeds. The transmission uses gears ranging from low to high to make more effective use of the engine’s torque as the driving conditions change. The two typical types of transmission are automatic and manual. The manual transmission uses a clutch and locks and unlocks different sets of gears to achieve the different gear ratios. The automatic transmission on the other hand has a torque convertor and planetary gears that allow the same set of gears to produce the different gear ratios.

Figure 1: Open-model automatic transmission
Harris, William Howstuffworks "Transmissions Basics". Retrieved March 3, 2007, from Howstuffworks Web site: http://auto.howstuffworks.com/cvt1.htm