Editing Overdrive (mechanics) (section)

==Description==
===Background===
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. These produce two primary forces slowing the car: [[rolling resistance]] and [[Drag (physics)|air drag]]. The former varies roughly with the speed of the vehicle, while the latter varies with the square of the speed. Calculating these from first principles is generally difficult due to a variety of real-world factors, so this is often measured directly in [[wind tunnel]]s and similar systems.

The power produced by an engine increases with the engine's RPM to a maximum, then falls away. This is known as the [[power band|point of maximum power]]. 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 defines the maximum speed the vehicle is able to reach. The rotational speed of the wheels for that given forward speed is simple to calculate, being the tire circumference multiplied by the RPM.{{Efn|For instance, a 15-inch wheel with 215/65 tyres has a diameter of about 26 inches, or a circumference of about 82 inches. At 100 mph, or 1760 inches per second, the wheel will be turning 21.5 times per second, or just under 1,300 RPM.}} As the tire RPM at maximum speed is not the same as the engine RPM at that power, a [[Transmission (mechanics)|transmission]] is used with a gear ratio to convert one to the other.{{Efn|Using the example above, at 100 mph the engine might need to be turning 5,000 RPM to generate the required power to turn the tyres at 1,300 RPM. A transmission with a gear ratio of 4:1 would be appropriate in this case.}}

At 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. In this case the RPM of the engine has changed significantly while the RPM of the wheels has changed very little. Clearly this condition calls for a different gear ratio. If one is not supplied, the engine is forced to run at a higher RPM than optimal. As the engine requires more power to overcome internal friction at higher RPM, this means more fuel is used simply to keep the engine running at this speed. Every cycle of the engine leads to wear, so keeping the engine at higher RPM is also unfavorable for engine life. Additionally, the sound of an engine is strongly related to the RPM, so running at lower RPM is generally quieter.<ref name="Setright, Anatomy of the Motor Car, Overdrive" />

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 fuel mileage. In an era when cars were not able to travel very fast, the maximum power point might be near enough to the desired speed that additional gears were not needed. But as more powerful cars appeared, especially during the 1960s, this disparity between the maximum power point and desired speed grew considerably. This meant that cars were often operating far from their most efficient point. As the desire for better [[fuel economy in automobiles|fuel economy]] grew, especially after the [[1973 oil crisis]], the need for a "cruising gear" became more pressing.<ref name="Setright, Anatomy of the Motor Car, Overdrive" />

===Gearbox vs. final drive===
The obvious solution to this problem would be to add more gears to the transmission. Indeed, in modern vehicles this is common. However, due to historical particularities, this was not always practical.

In the conventional [[Front-engine, rear-wheel drive layout|rear-wheel drive layout]], the transmission system normally contained two sections, the "gearbox" or "transmission" mounted behind the engine, and the "final drive" mounted in the [[rear axle]] at the rear of the car. The reason for this separation of duties between the front and back of the car was to allow the [[drive shaft]] to run at lower torque, by using higher RPM. As power is the product of RPM and [[torque]], running the shaft at higher RPM allowed more power to be transferred at lower torque. Doing so reduced the torque the driveshaft had to carry, and thus the strength and weight required.

Although the designer was theoretically free to choose any ratio for the gearbox and final drive, there is one additional consideration which meant that the top gear of most gearboxes was 1:1 or "direct drive". This is chosen for efficiency, as it does not require any gears to transmit power and so reduces the power lost by them. This was particularly important in the early days of cars, as their straight-cut gears were poorly finished, noisy and inefficient. The final drive then took this output and adjusted it in a fixed-ratio transmission arrangement that was much simpler to build. Final drive ratios of 4:1 were common,{{Efn|This ratio varies between cars, from around 3.5:1 to 5:1. American cars with large-slow-revving engines would use higher ratios, European compact cars with small high-revving engines were lower. Often the final drive ratio varied between models within a range, a "sports" model having a lower ratio.}} meaning that the wheels would turn at one fourth the rate they would if directly connected to the engine.

===Overdrive===
In an era when different models of car with different wheel sizes could be accommodated by simply changing the final drive ratio, it made sense for all transmissions to use direct drive as the highest gear. As noted earlier, however, this would cause the engine to operate at too high an RPM for efficient cruising. Although adding the cruising gear to the main gearbox was possible, it was generally simpler to add a separate two-gear overdrive system to the existing gearbox. This not only meant that it could be tuned for different vehicles, but had the additional advantage that it could be offered as an easily installed option.

With the use of front-wheel drive layouts, the gearbox and final drive are combined into a single transaxle. There is no longer a drive shaft between them and so the notion of "direct drive" is inapplicable. Although "overdrive" is still referred to, this is now mostly a marketing term to refer to any extra-high ratio for efficient cruising, whether it is achieved through the gearbox ratios, or by an unusually high final drive.{{Efn|Small Volkswagens of the 1980s, such as the [[Volkswagen Polo Mk2|Polo]], were marketed to an environmentally-conscious market with an overdrive top ratio labelled on the gear shift as "E", variously described as "Efficiency", "Economy" or "Environment".}}