Using the principle of mechanical advantage, transmissions provide a speed-torque conversion (commonly known as "gear reduction" or "speed reduction") from a higher speed motor to a slower but more forceful output or vice-versa.scion xb

Many typical automobile transmissions include the ability to select one of several different gear rations. In this case, most of the gear ratios (simply called "gears") are used to slow down the output speed of the engine and increase torque. However, the highest gears may be transmission"over drive" types that increase the output speed.xb

The gear box in a wind turbine transmission xb converts the slow, transmission xb high-torque rotation of the turbine into much faster rotation of the transmission xb electrical generator. These are much larger and more transmission xb complicated than the PTO gear boxes in farm equipment. They weigh several tons and typically contain three stages transmission xb to achieve an overall gear ratio from 40 to over 100, depending on the size of the turbine. (For aerodynamic and structural reasons, larger turbines have to turn more slowly, but the generators all have to rotate at similar speeds of several thousand rpm.) The first stage of the gear box is usually transmission xb   a planetary gear, for compactness, and to distribute the enormous torque of the turbine over more teeth of the low-speed shaft Durability of these gearboxes has been a serious problem for a transmission xb long time The need for a transmission xb in an automobile is a consequence of the characteristics of the internal combustion engine. Engines typically operate over a range of 600 to about 7000 revolutions per minute (though this varies, and is typically less for diesel engines), while the car's wheels rotate between 0 rpm and around 1800 rpm. transmission xb transmission Furthermore, the engine transmission xb provides its highest transmission xb torque outputs approximately in the middle transmission xb of its range, while often the greatest torque is required  transmission xb when the vehicle is moving from rest or traveling slowly. Therefore, a system that transforms the engine's output so that it can supply high torque at low xb speeds, but also operate at highway speeds with the motor still operating within its limits, is required. Transmissions perform this transformation.

• a simple but rugged sliding-mesh or unsynchronized non-synchromesh system, where straight-cut spur gear sets are spinning freely, and must be synchronized by the operator matching engine revs to road speed, to avoid noisy and damaging "gear clash",
• and the now common constant-mesh gearboxes transmission which can include non-synchronized, or  sun synchronized non-synchromesh systems, where diagonal cut helical (and sometimes double-helical) gear sets are constantly "meshed" together, and a dog clutch is used for changing gears. On synchromesh transmission boxes, friction cones or "synchro-rings" are used in addition to the dog clutch.