The gears of a truck, as seen between the fuel gage and speedometer
A gear was a mechanical device or a mechanical system that was part of a machine or device. Landing gear was a type of gear used for planetary landings.
With automobiles, gears referred to the various functions the vehicle was capable of. For example, switching to the gear marked with the letter "P"; which stood for "park," kept the vehicle in a stationary position.
Switching it to the gear marked with an "R"; which stood for "reverse," allowed the driver to drive the car backward. Switching to "N" meant neutral, and "D" meant drive in a forward position. Likewise the "2" referred to "second gear."
In 2004, the various gears of a Dodge Ram that Jonathan Archer and T'Pol had stolen were visible on the dash display between the gas gage and the speedometer. (ENT: "Carpenter Street")
In 2268, when James T. Kirk and Spock were using a flivver for transportation, Kirk noticed that the car had wheels and gears. At first he had trouble locating the starter, and Spock told him the clutch was likely one of the pedals on the floor. (TOS: "A Piece of the Action")