I want to knw how pneumatipneum operated gear shifting mechanism is dobe is off road vehiclr

Overdrive is a term used to describe the operation of a cruising car at a sustained speed with reduced revolutions per minute (RPM), which leads to better fuel consumption, lower noise and less wear. The use of the term is confusing as it applies to several different but related meanings.

The most fundamental meaning is that of a global gear ratio between the engine and the wheels, so that the car is over-geared, and can not reach its maximum potential speed, ie the car could travel faster if it was in A lower gear, with the engine turning at higher RPM.

The purpose of such a gear may not be immediately obvious. The power produced by an engine increases with the engine RPM to a maximum, then drops. The maximum power point is somewhat lower than the absolute maximum RPM to which the engine is limited, the RPM "redline". The speed of a car is limited by the power required to drive it against the resistance of the air, which increases with speed. At the maximum possible speed, the engine is running at its maximum power point, or peak power, and the car is traveling at the speed where the air resistance equals that maximum power. Therefore, there is a specific transmission ratio in which the car can reach its maximum speed: the one that matches that engine speed with that speed of movement. At shift speeds below this maximum, there is a range of gear ratios that can match engine power to air resistance, and the most fuel efficient is the one that results in the lowest engine speed. Therefore, a car needs a gear to achieve maximum speed but another to achieve maximum fuel efficiency at lower speed.

With the early development of cars and the almost universal design of rear-wheel drive, the final ratio (ie the rear axle) was chosen for fast cars to give maximum speed ratio. The gearbox was designed in such a way that, for efficiency, the fastest ratio would be a 1: 1 "direct drive" or "straight" ratio, avoiding frictional losses in the gears. Obtaining an overdrive ratio for the cruiser thus required an even greater gearbox ratio, ie the output shaft of the gearbox rotating faster than the engine. Therefore, the axis of the propeller connecting the gearbox and the rear axle is overmoted, and a transmission capable of doing so is referred to as "overdrive" transmission.

The device to achieve an overdrive transmission was usually a separate small gearbox, attached to the rear of the main gearbox and controlled by its own gearshift lever. These were often optional on some models of the same car.