In mechanics, a transmission or gearbox is the gear and/or hydraulic system that transmits mechanical power from a prime mover (which can be an engine or electric motor), to some form of useful output device.
Early transmissions (gearboxes) included right-angle drives and other gearing in windmills, horse-powered devices, and steam engines, mainly in support of pumping, milling, and hoisting.
Most modern gearboxes will either reduce an unsuitable high speed and low torque of the prime mover output shaft to a more useable lower speed with higher torque, or do the opposite and provide a mechanical advantage (i.e increase in torque) to allow higher forces to be generated. However, some of the simplest gearboxes merely change the physical direction in which power is transmitted.
Many systems, such as typical automobile transmissions, include the ability to select one of several different gear ratios. 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 gear(s) may be an "overdrive" type that increases the output speed.
Gearboxes have found use in a wide variety of different—often stationary—applications. Transmissions are also used in agricultural, industrial, construction, and mining equipment. In addition to ordinary transmission equipped with gears, such equipment makes extensive use of the hydrostatic drive and electrical Adjustable Speed Drives.
The simplest transmissions, often called gearboxes to reflect their simplicity (although complex systems are also called gearboxes on occasion), provide gear reduction (or, more rarely, an increase in speed), sometimes in conjunction with a right-angle change in direction of the shaft. These are often used on PTO-powered agricultural equipment, since the axial PTO shaft is at odds with the usual need for the driven shaft, which is either vertical (as with rotary mowers), or horizontally extending from one side of the implement to another (as with manure spreaders, flail mowers, and forage wagons). More complex equipment, such as silage choppers and snowblowers, have drives with outputs in more than one direction.
Regardless of where they are used, these simple transmissions all share an important feature: the gear ratio cannot be changed during use. It is fixed at the time the transmission is constructed.
Many applications require the availability of multiple gear ratios. Often, this is to ease the starting and stopping of a mechanical system, though another important need is that of maintaining good fuel economy.
The need for a transmission in an automobile is a consequence of the characteristics of the internal combustion engine. Engines typically operate over a range of 600 to about 6000 RPM (though this varies from design to design and is typically less for diesel engines), while the car's wheels rotate between 0 rpm and around 2500 RPM.
Furthermore, the engine provides its highest torque outputs approximately in the middle of its range, while often the greatest torque is required when the vehicle is moving from rest or travelling slowly. Therefore, a system that transforms the engine's output so that it can supply high torque at low speeds, but also operate at highway speeds with the motor still operating within its limits, is required. Transmissions perform this transformation.
Most transmissions and gears used in automotive and truck applications are contained in a cast iron case, though sometimes aluminum is used for lower weight. There are three shafts: a mainshaft, a countershaft, and an idler shaft.
The mainshaft extends outside the case in both directions: the input shaft towards the engine, and the output shaft towards the rear axle (on rear-wheel drive cars). The shaft is suspended by the main bearings, and is split towards the input end. At the point of the split, a pilot bearing holds the shafts together. The gears and clutches ride on the mainshaft, the gears being free to turn relative to the mainshaft except when engaged by the clutches.
- Main article: Manual transmission
Manual transmissions come in two basic types: a simple unsynchronized system where gears are spinning freely and must be synchronized by the operator to avoid noisy and damaging "gear clash", and synchronized systems that will automatically "mesh" while changing gears. The former type is only used on some rally cars nowadays.
Manual transmissions dominate the car market outside of North America. They are cheaper, lighter, usually give better performance and fuel efficiency (although the latest sophisticated automatic transmissions may yield results slightly closer to the ones yielded by manual transmissions), and it is customary for new drivers to learn, and be tested, on a car with a manual gearchange. In Germany, the UK, and France at least, a test pass using an automatic car does not entitle the driver to use a manual car on the public road unless a second manual test is taken. In most of the other European nations like Italy and the Netherlands, obtaining a driver's licence is only possible by passing a driver's test driving a car with manual transmission. Manual transmissions are much more common than automatic transmissions in Asia.
- Main article: Automatic transmission
Most modern North American cars have an automatic transmission that will select an appropriate gear ratio without any operator intervention. They are primarily using hydraulics to select gears, depending on pressure exerted by fluid within the transmission assembly. Rather than using a clutch to engage the transmission, a torque converter is put in between the engine and transmission. It is possible for the driver to control the number of gears in use or select reverse, though precise control of which gear is in use is usually not possible.
Automatic transmissions are easy to use. In the past, automatic transmissions of this type have had a number of problems; they were complex and expensive, sometimes had reliability problems (which sometimes caused more expenses in repair), and have often been less fuel-efficient than their manual counterparts. With the advancement of modern automatic transmissions this has changed. With computer technology, considerable effort has been put into designing gearboxes based on the simpler manual systems that use electronically-controlled actuators to shift gears and manipulate the clutch, resolving many of the drawbacks of a hydraulic automatic transmission.
Automatic transmissions have always been extremely popular in the United States, where perhaps 19 of 20 new cars are sold with them (many vehicles are not available with manual gearboxes anymore). In Europe automatic transmissions are gaining popularity as well.
Attempts to improve the fuel efficiency of automatic transmissions include the use of torque converters which lock-up beyond a certain speed eliminating power loss, and overdrive gears which automatically actuate above certain speeds; in older transmissions both technologies could sometimes become intrusive, when conditions are such that they constantly cut in and out as speed and such load factors as grade or wind vary slightly. Current computerized transmissions possess very complex programming to both maximize fuel efficiency and eliminate any intrusiveness.
For certain applications, the slippage inherent in automatic transmissions can be advantageous; for instance, in drag racing, the automatic transmission allows the car to be stopped with the engine at a high rpm (the "stall speed") to allow for a very quick launch when the brakes are released; in fact, a common modification is to increase the stall speed of the transmission. This is even more advantageous for turbocharged engines, where the turbocharger needs to be kept spinning at high rpm by a large flow of exhaust in order to keep the boost pressure up and eliminate the turbo lag that occurs when the engine is idling and the throttle is suddenly opened.
- Main article: Semi-automatic transmission
The creation of computer control also allowed for a sort of half-breed transmission where the car handles manipulation of the clutch automatically, but the driver can still select the gear manually if desired. This is sometimes called "clutchless manual". Many of these transmissions allow the driver to give full control to the computer.
There are also sequential transmissions which use the rotation of a drum to switch gears. A great example of this is the 7-speed sequential transmission on the [[Bugatti_Veyron_16.4|Bugatti Veyron]], a supercar that puts out 1,001 horsepower (746 kW) and goes 254 miles per hour (409 km/h). 
Bicycles usually have a system for selecting different gear ratios as well. There are two main types, derailleur gears and hub gears. The derailleur type is the most common, and the most visible, using a number of sprocket gears. Typically there are several gears available on the rear sprocket assembly, attached to the rear wheel. A few more sprockets are usually added to the front assembly as well. Multiplying the number of sprocket gears in front with the number to the rear gives the number of different gear ratios, often called "speeds". A 21-speed bike will have three sprocket wheels in front and seven in back.
Hub gears use epicyclic gearing and are enclosed within the axle of the rear wheel. Because of the small space, they typically only offer a handful of different speeds, although at least one has reached the level of 14 different gear ratios.
- Main article: Continuously variable transmission
The mechanical systems described above only allow a few different gear ratios to be selected, but there does exist a type of transmission that essentially has an infinite number of ratios available. The continuously variable transmission allows the relationship between the speed of the engine and the speed of the wheels to be varied constantly. This can provide even better fuel economy if the engine is constantly running at a single speed. However, this is somewhat disconcerting to drivers, who are accustomed to hearing and feeling the rise and fall in speed of an engine, and the "jerk" felt when changing gears. Changes to software in the computer control system can simulate these effects, however.
Hydrostatic transmissions transmit all power with hydraulics; there is no mechanical coupling of the input and output. One half of the transmission is a variable displacement pump and the other half is a hydraulic motor. A movable swash plate controls the piston stroke to change the pump's displacement.
They are used in the drivetrain of some types of heavy equipment, diesel multiple unit trains, and applications requiring continuously variable control (such as riding lawnmowers and lawn tractors). Their disadvantages are high cost and sensitivity to contamination.
Electric transmissions convert the mechanical power of the engine(s) to electricity with electric generators and convert it back to mechanical power with electric motors. If the generators are driven by turbines, such arrangements are called turbo-electric. Likewise installations powered by diesel-engines are called diesel-electric.