The propeller shaft linking gearbox and rear axle is thus overdriven, and a transmission capable of doing this became termed an "overdrive" transmission. the gearbox output shaft rotating faster than the original engine RPM. Achieving an overdriven ratio for cruising thus required a gearbox ratio even higher than this, i.e. The gearbox was designed so that, for efficiency, the fastest ratio would be a "direct-drive" or "straight-through" 1:1 ratio, avoiding frictional losses in the gears. rear axle) ratio for fast cars was chosen to give the ratio for maximum speed. With the early development of cars and the almost universal rear-wheel drive layout, the final drive (i.e. This power is available well below the engine's power peak and so the ideal cruising gear is an overdrive gear, a ratio higher than that for absolute top speed. As drag causes the power needed to increase with the cube of the velocity (v 3), most cars will be capable of achieving a fast cruising speed less than their maximum, with far less power being required. A car's speed is limited by the power available to drive it against air resistance-so the maximum possible speed is obtained at the engine's point of maximum power, or power peak, and the gear ratio necessary to achieve this will be the single ratio between these two speeds. The point of maximum power is somewhat slower than the absolute maximum RPM to which the engine is limited, the " redline". The power produced by an engine increases with the engine's RPM to a maximum, then falls away. The purpose of such a gear may not be immediately obvious. the car could travel faster if it were in a lower gear, with the engine turning more quickly. The most fundamental meaning is that of an overall gear ratio between engine and wheels, such that the car is now over-geared and can no longer reach its potential top speed, i.e. Use of the term is confused, as it is applied to several different, but related, meanings. Overdrive (OD) is a term used to describe a mechanism that allows an automobile to cruise at sustained speed with reduced engine RPM, leading to better fuel economy, lower noise and lower wear. Laycock de Normanville "J type" Overdrive Unit.