Overdrive is a term used to describe a mechanism that allows an automobile

Automobile

An automobile, autocar, motor car or car is a wheeled motor vehicle used for transporting passengers, which also carries its own engine or motor...

 to cruise at sustained speed with reduced engine RPM

Revolutions per minute

Revolutions per minute is a measure of the frequency of a rotation. It annotates the number of full rotations completed in one minute around a fixed axis...

, leading to better fuel economy, lower noise and lower wear. Use of the term is confused, as it is applied to several different, but related, meanings.

The most fundamental meaning is that of an overall gear ratio

Gear ratio

The gear ratio of a gear train is the ratio of the angular velocity of the input gear to the angular velocity of the output gear, also known as the speed ratio of the gear train. The gear ratio can be computed directly from the numbers of teeth of the various gears that engage to form the gear...

 between engine and wheels, such that the car is now over-geared and can no longer reach its potential top speed, i.e. the car could travel faster if it were in a lower gear, with the engine turning more quickly. The purpose of such a gear may not be immediately obvious. The power produced by an engine increases with the engine's RPM to a maximum, then falls away. The point of maximum power

Power band

The power band of an engine or electric motor refers to the range of operating speeds under which an the engine or motor is able to operate efficiently...

 is somewhat slower than the absolute maximum RPM to which the engine is limited, the "redline

Redline

Redline refers to the maximum engine speed at which an internal combustion engine or traction motor and its components are designed to operate without causing damage to the components themselves or other parts of the engine...

". 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. As drag causes the power needed to increase with the cube of the velocity (v³), most cars will be capable of achieving a fast cruising speed less than their maximum, with far less power being required. 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.

With the early development of cars and the almost universal rear-wheel drive layout, the final drive (i.e. rear axle) ratio for fast cars was chosen to give the ratio for maximum speed. 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. Achieving an overdriven ratio for cruising thus required a gearbox ratio even higher than this, i.e. the gearbox output shaft rotating faster than the original engine RPM. The propeller shaft linking gearbox and rear axle is thus overdriven, and a transmission

Transmission (mechanics)

A machine consists of a power source and a power transmission system, which provides controlled application of the power. Merriam-Webster defines transmission as: an assembly of parts including the speed-changing gears and the propeller shaft by which the power is transmitted from an engine to a...

 capable of doing this became termed an "overdrive" transmission.

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. These were often an optional extra on some models of the same car.

As popular cars became faster relative to legal limits and fuel costs became more important, particularly after the 1973 oil crisis

1973 oil crisis

The 1973 oil crisis started in October 1973, when the members of Organization of Arab Petroleum Exporting Countries or the OAPEC proclaimed an oil embargo. This was "in response to the U.S. decision to re-supply the Israeli military" during the Yom Kippur war. It lasted until March 1974. With the...

, the use of 5-speed gearboxes became more common in mass-market cars. These had a direct fourth speed with an overdrive 5th gear, replacing the need for the separate overdrive gearbox.

With the popularity of front wheel drive cars, the separate gearbox and final drive have merged into a single transaxle

Transaxle

In the automotive field, a transaxle is a major mechanical component that combines the functionality of the transmission, the differential, and associated components of the driven axle into one integrated assembly....

. There is no longer a propeller shaft and so one meaning of "overdrive" can no longer be applied. However the fundamental meaning, that of an overall ratio higher than the ratio for maximum speed, still applies. Although the deliberate labelling of an overdrive is now rare, the underlying feature is now found across all cars.

Description

The transmission of a car reduces the rotational speed of the drivetrain from that appropriate to the engine to that suiting the wheels. It also allows this ratio to change when shifting gear, so that the best ratio is in force for the road speed, keeping the engine RPM roughly constant. At top speed, this ratio might be around 4:1 with far higher ratios in the lower gears.

In the widely-used rear-wheel drive layout

Front-engine, rear-wheel drive layout

In automotive design, an FR, or Front-engine, Rear-wheel drive layout is one where the engine is located at the front of the vehicle and driven wheels are located at the rear. This was the traditional automobile layout for most of the 20th century....

, the transmission consisted of two parts: a "gearbox" that provided the additional reduction in the lower gears and also allowed gear shifting (manual or automatic) and also a "final drive" that provided a 4:1 (approximately) reduction ratio that was always in effect. The gearbox was usually mounted forwards, next to the engine, and the final drive was mounted on the rear axle where it also housed the bevel drive gears to change the direction of rotation and the differential. To allow a lightweight drive shaft to be used to connect these, its rotational speed was kept high, thus reducing the torque it had to carry and thus the strength and weight it required. For this reason, the final drive retained the last step of speed reduction and this was not performed by the gearbox.

The top ratio of a non-overdrive gearbox is 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. As techniques of gear-cutting improved, the losses of a gear train reduced and it became practical to use an overdrive gearbox. This usually still offered the same direct ratio, but also had an additional ratio even higher, so that the output speed was faster than the input engine speed. Mechanically this had the same effect as installing a higher final drive ratio, but was simpler to provide.

In practice, the use of a separate overdrive gearbox was popular, mounted as an extension to the rear of the main gearbox. This was often an optional extra, or limited to the top models in a range, the standard gearbox being supplied without. This gearbox was also often electro-hydraulically activated by a simple switch on the dashboard. At a time when gear changing was generally slow and heavy, this easy shift for overtaking at high speed was popular just for its convenience.

History

Early manual automotive transmissions were limited to three or sometimes four speeds, with only the most sophisticated being five speeds. This left an unfulfilled need for a higher gear ratio for highway cruising, which was filled by the addition of separate overdrive units (auxiliary or bolt-on), in order to simulate an extra gear. Today, however, automotive transmissions manufactured since the 1980s tend to include overdrive within the transmission. In the aftermarket, early non-overdrive vehicles and 3/4 ton or heavier diesel trucks (where fuel economy and more gears are always in demand) are the main use of bolt-on auxiliary overdrives.

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" ratios is simply 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.

Usage

Generally speaking, overdrive is the highest gear in the transmission. Overdrive allows the engine to operate at a lower RPM for a given road speed. This allows the vehicle to achieve better fuel efficiency, and often quieter operation on the highway. When it is switched on, an automatic transmission can shift into overdrive mode after a certain speed is reached (usually 70+ km/h [40-45 mph or more] depending on the load). When it is off, the automatic transmission shifting is limited to the lower gears. For an automatic transmission, it is almost always best to select overdrive and allow the transmission to control engagement of the overdrive. (It may be necessary to switch it off if the vehicle is being operated in a mountainous area or when towing a trailer.) With a manual transmission, overdrive should usually be selected when the average speed is above 70 km/h (40-45 mph).

The automatic transmission automatically shifts from OD to direct drive when more load is present. When less load is present, it shifts back to OD. Under certain conditions, for example driving uphill, or towing a trailer, the transmission may "hunt" between OD and the third gear, shifting back and forth. In this case, switching it off can help the transmission to "decide". It may also be advantageous to switch it off if engine braking

Engine braking

Engine braking is where the retarding forces within an engine are used to slow a vehicle down, as opposed to using an external braking mechanism, for example friction brakes or magnetic brakes....

 is desired, for example when driving downhill. The vehicle's owner's manual will often contain information and suitable procedures regarding such situations, for each given vehicle.

Virtually all vehicles (cars and trucks) have overdrive today whether manual transmission or automatic. In the automotive aftermarket you can also retrofit overdrive to existing early transmissions. Overdrive was widely used in European automobiles with manual transmission in the 60s and 70s to improve mileage and sport driving as a bolt-on option but it became increasingly more common for later transmissions to have this gear built in. If a vehicle is equipped with a bolt-on overdrive (e.g.: GKN or Gear Vendors) as opposed to a having overdrive built in one will typically have the option to use the overdrive in more gears than just the top gear. In this case gear changing is still possible in all gears, even with overdrive disconnected. Overdrive simply adds effective ranges to the gears, thus overdrive third and fourth become in effect "third-and-a-half" and a fifth gear. In practice this gives the driver more ratios which are closer together providing greater flexibility particularly in performance cars.

How an overdrive unit works

The overdrive consists of an electrically or hydraulically operated epicyclic gear

Epicyclic gearing

Epicyclic gearing or planetary gearing is a gear system consisting of one or more outer gears, or planet gears, revolving about a central, or sun gear. Typically, the planet gears are mounted on a movable arm or carrier which itself may rotate relative to the sun gear...

 train bolted behind the transmission

Transmission (mechanics)

A machine consists of a power source and a power transmission system, which provides controlled application of the power. Merriam-Webster defines transmission as: an assembly of parts including the speed-changing gears and the propeller shaft by which the power is transmitted from an engine to a...

 unit. It can either couple the input driveshaft

Driveshaft

A drive shaft, driveshaft, driving shaft, propeller shaft, or Cardan shaft is a mechanical component for transmitting torque and rotation, usually used to connect other components of a drive train that cannot be connected directly because of distance or the need to allow for relative movement...

 directly to the output shaft (or propeller shaft) (1:1), or increase the output speed so that it turns faster than the input shaft (1:1 + n). Thus the output shaft may be "overdriven" relative to the input shaft. In newer transmissions, the overdrive speed(s) are typically as a result of combinations of planetary/epicyclic gearsets which are integrated in the transmission. In these cases, there is no separately identifiable "overdrive" unit. A number of such transmissions and transaxles are manufactured by Aisin

Aisin Seiki Co.

, also known as Aisin , is a Japanese corporation which develops and produces components and systems for the automotive industry. Aisin is a Fortune Global 500 company, ranked 347 on the 2007 rankings....

, for use in vehicles produced by many different manufacturers. In older vehicles, it is sometimes actuated by a knob or button, often incorporated into the gearshift knob, and does not require operation of the clutch

Clutch

A clutch is a mechanical device which provides for the transmission of power from one component to another...

. Newer vehicles have electronic overdrive in which the computer automatically adjusts to the conditions of power need and load.

Overdrive in Europe

The vast majority of overdrives in Europe

Europe

Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...

an cars were manufactured by an English

United Kingdom

The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...

 company called Laycock de Normanville (later GKN

GKN

GKN plc is a multinational automotive and aerospace components company headquartered in Redditch, United Kingdom. The company was formerly known as Guest, Keen and Nettlefolds and can trace its origins back to 1759 and the birth of the Industrial Revolution.GKN is listed on the London Stock...

 Laycock), at its Little London Road site in Sheffield, which is now demolished and remanufactured in the UK by an ex-Laycock de Normanville employee trading as Overdrive Spares. The system was devised by a Briton, named Captain Edgar J de Normanville (1884–1968), through a chance meeting with a Laycock Products Engineer. De Normanville overdrives were found in vehicles manufactured by Ford

Ford Motor Company

Ford Motor Company is an American multinational automaker based in Dearborn, Michigan, a suburb of Detroit. The automaker was founded by Henry Ford and incorporated on June 16, 1903. In addition to the Ford and Lincoln brands, Ford also owns a small stake in Mazda in Japan and Aston Martin in the UK...

, British Leyland, Jaguar

Jaguar (car)

Jaguar Cars Ltd, known simply as Jaguar , is a British luxury car manufacturer, headquartered in Whitley, Coventry, England. It is part of the Jaguar Land Rover business, a subsidiary of the Indian company Tata Motors....

, Rootes Group and Volvo

Volvo Cars

Volvo Car Corporation, or Volvo Personvagnar AB, is a Swedish automobile manufacturer founded in 1927, in Gothenburg, Sweden. It is owned by Zhejiang Geely Holding Group. Volvo was originally formed as a subsidiary company to the ball bearing maker SKF. When Volvo AB was introduced on the Swedish...

 to name a few. Another British company, the former aircraft builder Fairey

Fairey

-People:*Charles Richard Fairey, British aircraft manufacturer*Francis Fairey, , Canadian politician,*Jim Fairey, outfielder*Shepard Fairey, American artist-Companies:*Fairey Aviation Company, British aircraft company...

, built a successful all-mechanical unit for the Land Rover

Land Rover

Land Rover is a British car manufacturer with its headquarters in Gaydon, Warwickshire, United Kingdom which specialises in four-wheel-drive vehicles. It is owned by the Indian company Tata Motors, forming part of their Jaguar Land Rover group...

, which is still in production in America today.

The first production vehicle to feature the Laycock system was the 1948 Standard Vanguard Saloon. The first unit to be created was the A-type overdrive, this was fitted to many sports cars during the 1950s. Several famous marques used A-type overdrives, including Jaguar, Aston Martin, Ferrari, Austin-Healey, Jensen, Bristol, AC and Armstrong Siddeley. Later the A-type overdrive was to be fitted to the entire range of TR cars.

In 1959, the Laycock Engineering Company introduced the D-type overdrive, which was fitted to a variety of motor cars including Volvo 120 and 1800s, Sunbeam Alpine

Sunbeam Alpine

The Sunbeam Alpine is a sporty two-seat open car from Rootes Group's Sunbeam car marque. The original was launched in 1953 as the first vehicle from Sunbeam-Talbot to bear the Sunbeam name alone since the 1935 takeover of Sunbeam and Talbot by the Rootes Group....

s and Rapiers

Sunbeam Rapier

The Series I Rapier was a pretty car which in general, was well received by the motoring press. Available in a range of attractive two-tone colour schemes typical of the period, it boasted steering column gear change, leather trim and an overdrive as standard fittings. Vinyl trim was an option in...

, Triumph Spitfire

Triumph Spitfire

The Triumph Spitfire is a small English two-seat sports car, introduced at the London Motor Show in 1962. The vehicle was based on a design produced for Standard-Triumph in 1957 by Italian designer Giovanni Michelotti...

s, and also 1962-1967 MGB

MG MGB

The MGB is a sports car launched by MG Cars in May 1962 to replace the MGA. Introduced as a four-cylinder roadster, a coupé with 2+2 seating was added in 1965...

s (those with 3-synchro transmissions).

From 1967 the LH-type overdrive was introduced, and this featured in a variety of models, including 1968-1980 MGB

MG MGB

The MGB is a sports car launched by MG Cars in May 1962 to replace the MGA. Introduced as a four-cylinder roadster, a coupé with 2+2 seating was added in 1965...

s, the MGC, the Ford Zephyr

Ford Zephyr

The Ford Zephyr was a car manufactured by the Ford Motor Company in the United Kingdom. Between 1950 and 1972, it was sold as a more powerful six-cylinder saloon to complement the four-cylinder Ford Consul: from 1962 the Zephyr itself was offered in both four- and six-cylinder versions.The Zephyr...

, early Reliant Scimitar

Reliant Scimitar

Reliant's first Scimitar was a coupé based upon the styling of a Daimler SP250 prototype and the chassis of a Reliant Sabre. It was first displayed in 1964. It was powered by a 2.6 L Ford straight six from the Ford Zephyr / Ford Zodiac...

s, TVRs, and Gilbern

Gilbern

Gilbern cars were made in Llantwit Fardre, Pontypridd, Glamorgan, Wales between 1959 and 1973.Gilbern Sports Cars Ltd was founded by Giles Smith, a butcher, and Bernard Friese, a German engineer with experience in glass fibre mouldings, and was one of the few cars to be made in Wales...

s.

The J-type overdrive was introduced in the early 1970s, and was adapted to fit Volvo, Triumph, Vauxhall/Opel, American Motors and Chrysler motorcars, and Ford Transit

Ford Transit

The Ford Transit is a range of panel vans, minibuses, and pickup trucks, produced by the Ford Motor Company in Europe.The Transit has been the best-selling light commercial vehicle in Europe for 40 years, and in some countries the term "Transit" has passed into common usage as a generic term...

 vans.

The P-type overdrive marked the last updates and included both a Gear Vendors U.S. version and a Volvo version. The Volvo version kept the same package size as the J-type but with the updated 18 element freewheel

Freewheel

thumb|Freewheel mechanismIn mechanical or automotive engineering, a freewheel or overrunning clutch is a device in a transmission that disengages the driveshaft from the driven shaft when the driven shaft rotates faster than the driveshaft...

 and stronger splines through the planet carrier. The Gear Vendors U.S. version uses a larger 1.375 outer diameter output shaft for higher capacity and a longer rear case.

Over a period of 40 years, Laycock Engineering manufactured over three and a half million overdrive Units, and over one million of these were fitted to Volvo motorcars.

In 2008 the U.S. company Gear Vendors, Inc. of El Cajon, California

El Cajon, California

-History:El Cajon is located on the Rancho El Cajon Mexican land grant made in 1845 to María Antonia Estudillo, wife of Miguel Pedrorena. In 1876 Amaziah Lord Knox , a New Englander who had recently moved to California, established a hotel there to serve the growing number of people traveling...

 purchased all the overdrive assets of GKN to continue production of the U.S. version and all spares for J and P types worldwide.

The system features an oil pressure operated device attached to the back of the standard gearbox operating on the gearbox output shaft. Through a system of oil pressure, solenoids and pistons, the overdrive would drop the revs on whatever gears it was used on by 22%. For instance, the overdrive system applied to a Triumph TR5

Triumph TR5

The Triumph TR5 was built for a 13 month period between August 1967 and September 1968 by the Triumph Motor Company in Coventry, England....

 operates on 2nd, 3rd and top gear. When engaged, the overdrive would drop the revs by approximately 450 RPM. The advantages this had on fuel consumption was quite marked over long distances.

Overdrive in North America

In the days before automatic transmissions were common, especially in the 1950s, many rear-wheel drive American cars were available with an overdrive option. Borg Warner provided the box that was factory-installed between the transmission and a foreshortened driveshaft. Since the overdrive function, if enabled, could be shifted by simply easing up on the accelerator without depressing the clutch pedal, the action was much like a semi-automatic. Also, an electrically operated solenoid would deactivate the unit via a switch under the accelerator pedal providing the equivalent of the kickdown of the automatic. A knob connected to a bowden cable

Bowden cable

A Bowden cable is a type of flexible cable used to transmit mechanical force or energy by the movement of an inner cable relative to a hollow outer cable housing...

, similar to some emergency brake applications, was also provided to lock out the unit mechanically. Using overdrive with the main 3-speed transmission in 2nd gear was similar in ratio to 3rd gear, and with the main transmission in 3rd, the overall ratio was fractional (i.e., "true overdrive").

Such add-on overdrive boxes were available from the 1930s to the 1970s for cars and light trucks.

Today, most gas and diesel cars and trucks come with an overdrive transmission because of the benefit to fuel economy. Overdrive is included in both automatic and manual transmissions as an extra gear (or two in some cases).

Since 1979, add-on overdrives are provided by Gear Vendors, Inc., El Cajon CA. for most U.S. made vehicles from the 1920s through to current (2010) cars and light trucks.

Fuel economy and drivetrain wear

When using overdrive gearing, the car's engine speed drops, reducing wear and normally saving fuel. Since 1981 U.S. corporate average fuel economy

Corporate Average Fuel Economy

The Corporate Average Fuel Economy are regulations in the United States, first enacted by the U.S. Congress in 1975, and intended to improve the average fuel economy of cars and light trucks sold in the US in the wake of the 1973 Arab Oil Embargo...

 (CAFE) legislation on virtually all domestic vehicles have included overdrive to save fuel. One should refer to the car's owner's manual for the proper speed to run at overdrive. All engines have a range of peak efficiency and it is possible for the use of overdrive to keep the engine out of this range for all or part of the time of its use if used at inappropriate speeds, thus cutting into any fuel savings from the lower engine speed.

There is some debate on the overall efficiency of overdrive gearing, as it requires more moving parts than direct 1:1 drive, but most will agree that within the transmission, this effect is minimal. The other difficulty can be in the drive shaft rotation speed.

Overall drivetrain reduction comes down to three basic factors: transmission gearing (including overdrive), differential gearing (in the axle), and tire size. The rotation speed problem comes into effect when the differential gearing is a high ratio and an overdrive is used to compensate. This may create unpleasant vibrations at high speeds and possible destruction of the driveshaft due to the centripetal forces or uneven balance.

The driveshaft is usually a hollow metal tube that requires balancing to reduce vibration and contains no internal bracing.

The higher speeds on the driveshaft and related parts can cause heat and wear problems if an overdrive and high differential gearing (or even very small tires) are combined, and create unnecessary friction. This is especially important because the differential gears are bathed in heavy oil and seldom provided with any cooling besides air blowing over the housing.

The impetus is to minimize overdrive use and provide a higher ratio first gear, which means more gears between the first and the last to keep the engine at its most efficient speed. This is part of the reason that modern automobiles tend to have larger numbers of gears in their transmissions. It is also why more than one overdrive gear is seldom seen in a vehicle except in special circumstances i.e. where high (numerical) diff gear is required to get the vehicle moving as in trucks or performance cars though double overdrive transmissions are common in other vehicles, often with a small number on the axle gear reduction, but usually only engage at speeds exceeding 100 km/h or about 60 mph.

External links

• "How Automatic Transmissions Work - Overdrive" (with a Flash interactive animation
  Animation
  Animation is the rapid display of a sequence of images of 2-D or 3-D artwork or model positions in order to create an illusion of movement. The effect is an optical illusion of motion due to the phenomenon of persistence of vision, and can be created and demonstrated in several ways...
  
  )