If one runs the same RPM transmission exercise outlined above for maximum speed, but instead sets the "maximum speed" to that of highway cruising, the output is a higher gear ratio that provides ideal gas milage. Additionally, the sound of an engine is strongly related to the RPM, so running at lower RPM is generally quieter. Every cycle of the engine leads to wear, so keeping the engine at higher RPM is also bad for engine life. As the engine requires more power to overcome internal friction at higher RPM, this means more gas is used simply to keep the engine running at this speed. If one is not supplied, the engine is forced to run at a higher RPM than optimal. Clearly this condition calls for a different gear ratio. In this case the RPM of the engine has changed significantly while the RPM of the wheels has changed very little. Īt even slightly lower speeds than maximum, the total drag on the vehicle is considerably less, and the engine needs to deliver this greatly reduced amount of power. As the tire RPM at maximum speed is not the same as the engine RPM at that power, a transmission is used with a gear ratio to convert one to the other. The rotational speed of the wheels for that given forward speed is simple to calculate, it is simply the tire diameter times the RPM. This defines the maximum speed the vehicle is able to reach. Given a curve describing the overall drag on the vehicle, it is simple to find the speed at which the total drag forces are the same as the maximum power of the engine. This is known as the point of maximum power. The power produced by an engine increases with the engine's speed to a maximum, then falls away. Calculating these from first principles is generally difficult due to a variety of real-world factors, so this is often measured directly in wind tunnels and similar systems. The former varies roughly with the speed of the vehicle, while the latter varies with the square of the speed, resulting in an overall relationship that varies roughly with the cube of speed. These produce two primary forces slowing the car, rolling resistance and air drag. The power needed to propel a car at any given set of conditions and speed is straightforward to calculate, based primarily on the total weight and the vehicle's speed. Although the deliberate labelling of an overdrive is now rare, the underlying feature is now found across all cars. However the fundamental meaning, that of an overall ratio higher than the ratio for maximum speed, still applies. With the popularity of front wheel drive cars, the separate gearbox and final drive have merged into a single transaxle. As popular cars became faster relative to legal limits and fuel costs became more important, particularly after the 1973 oil crisis, the use of 5-speed gearboxes became more common in mass-market cars, with the 5th gear being an overdrive, eliminating the need for a separate gearbox. These were often an optional extra on some models of the same car. In the era of front-engine, rear-wheel drive layouts, the device for achieving an overdrive transmission was usually a small separate gearbox, attached to the rear of the main gearbox and controlled by its own shift lever or electrical actuation button. This later definition was common in the past, but many cars no longer have a driveshaft and it is not technically correct. In some cases this implies a second definition is also true, that the gearbox output driveshaft is rotating faster than the original engine RPM. 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.
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