A preselector gearbox is a type of manual transmission mostly used on passenger cars and racing cars in the 1930s, in buses from 1940–1960 and in armoured vehicles from the 1930s to the 1970s. The defining characteristic of a preselector gearbox is that the gear shift lever allowed the driver to "pre-select" the next gear, usually with the transmission remaining in the current gear until the driver pressed the "gear change pedal" at the desired time.

The design removed the need for the driver to master the timing of using a clutch pedal and shift lever in order to achieve a smooth shift in a non-synchromesh manual transmission. Most pre-selector transmissions avoid a driver-controlled clutch entirely. Some use one solely for starting from a standstill. Preselector gearboxes were most common prior to the widespread adoption of the automatic transmission, so they were considered in comparison to the "crash gearbox" type of manual transmission.

Preselector gearboxes were often marketed as "self-changing" gearboxes, however this is an inaccurate description as the driver is required to choose the gear (and often manually actuate the gear change). An automatic transmission is a true "self-changing gearbox" since it is able to change gears without any driver involvement.

There are several radically different mechanical designs of preselector gearbox. The best known is the Wilson design. Some gearboxes, such as the Cotal, shift gears immediately as the control is moved, without requiring the separate gear change pedal.

Compared with the contemporary (non-synchromesh) manual transmissions, preselector gearboxes were easier for drivers to operate smoothly, since they did not require techniques such as double de-clutching. Preselector gearboxes also had faster shift times, could handle greater power outputs and had less mass and could shift under load.^{[citation needed]}

A design advantage of many preselector gearboxes is that the friction components are brakes, rather than clutches. This meant that non-rotating brake bands could be used for the parts which are subject to wear, which results in a simpler design than a rotating component such as a typical clutch. The wearing components could also be mounted on the outside of the mechanism (rather than buried within it), providing easier access for maintenance and adjustment.

The most common type of pre-selector gearbox was the Wilson, which used an epicyclic design. A precursor to the Wilson gearbox was the manually-controlled epicyclic gearbox used in the 1901–1904 Wilson-Pilcher cars built in the United Kingdom. One of the company directors, Walter Gordon Wilson, had become an advocate for the benefits of the epicyclic gearbox, which allowed large torques to be transmitted whilst still being controllable through a small input force. Walter Wilson was a major co-inventor of the armoured tank during and after World War I, and was responsible for the 1918 British Mark V tank using an epicyclic steering gearbox. The Lanchester Motor Company in the United Kingdom also produced cars with manually-controlled epicyclic gearboxes from 1900 and built an experimental tank (the Lanchester Gearbox Machine or Experimental Machine K) fitted with an epicyclic gearbox.

Walter Wilson continued experimentation with epicyclic gearboxes for cars and in 1928 his "Wilson gearbox" was invented. Walter and one of the co-owners of Armstrong Siddeley Motors formed the Self-Changing Gears Ltd. company (initially called Improved Gears Ltd.) to design and build the Wilson gearbox. The 1929 Armstrong Siddeley Thirty was one of the first cars to use the Wilson gearbox. The gearbox was also built under licence by other manufacturers including Cord, ERA, Daimler and Maybach.