Editing Manual transmission (section)

== External overdrive ==
{{Main article|overdrive (mechanics)|l1=Overdrive}}
In the 1950s, 1960s, and 1970s, fuel-efficient highway cruising with low engine speed was in some cases enabled on vehicles equipped with 3- or 4-speed transmissions by means of a separate [[overdrive (mechanics)|overdrive]] unit in or behind the rear housing of the transmission. This was actuated either manually while in high gear by throwing a switch or pressing a button on the gearshift knob or on the steering column, or automatically by momentarily lifting the foot from the accelerator with the vehicle traveling above a certain road speed. Automatic overdrives were disengaged by flooring the accelerator, and a lockout control was provided to enable the driver to disable overdrive and operate the transmission as a normal (non-overdrive) transmission.<ref>{{cite web|url=http://www.fordification.com/tech/overdrive.htm |title=The Borg-Warner Overdrive Transmission Explained |publisher=FORDification.com |access-date=2012-04-22}}</ref>

The term 'overdrive' is also used to describe a gear with a ratio of less than one (e.g., if the top gear of the transmission has a ratio of 0.8:1).