Mechanism in a manual gearbox to make gear changing easier by ensuring matching gearwheels rotate at the same speed before engaging.

an automotive system for shifting gears in which the gears revolve at the same speed and so shift smoothly

A device in a gearbox, which synchronises the speed of one gear, shaft with another to produce smooth, noiseless engagement of the gears. achometer - Also known as a rev counter, indicates engine speed in revolutions per minute (rpm).