Understanding Your Transmission, Part 2: Automatic Transmissions Posted on by in If you haven’t read my entry on, you may want to give that a brief glance, as it provides some useful background information on the basic ideas behind an automotive transmission. Some of these basic principles are shared by both automatic and manual transmissions. For example, both use different gear ratios to keep the engine’s output within its ideal RPM range as the car accelerates and decelerates. Unlike a manual transmission, in which the driver selects gears himself, the automatic transmission has only one “drive” setting. As the driver accelerates, the transmission shifts automatically through the different gears. In a manual transmission, a driver selects different gear ratios, i.e. There is a first gear, a second gear, etc.
A manual transmission, also known as a manual gearbox, stick shift, n-speed manual standard, MT, or in colloquial U.S. English, a stick is a type of transmission used in motor vehicle applications. It uses a driver-operated clutch engaged and disengaged by a foot pedal (automobile) or hand lever (motorcycle), for regulating.
- In a manual transmission, gears slide along shafts as you move the shift lever from one position to another, engaging various sized gears as required in order to provide the correct gear ratio. The basic planetary gear set consists of a sun gear, a ring gear and two or more planet gears, all remaining in constant mesh.
- 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.
In an automatic transmission, however, all of the gear ratios are produced by an ingenious device called a planetary gearset. However, an automatic transmission uses the same basic gear ratios as a manual transmission.
You have first, second, third, and overdrive gears, neutral (engine idles but is disengaged from transmission) and reverse. On rear wheel drives, the transmission is usually mounted at the back of the engine and connected to the wheels by a long driveshaft. In a front wheel drive, the transmission is combined with the final drive to form something called a “transaxle,” mounted under and to the side of the engine. These are two of the most common arrangements, but there are others as well. On newer cars, gear shifts are determined and controlled by a computer. However, the earliest known version of an automatic transmission was developed in 1904—long before the digital age—while the basis of our modern automatic transmission was full developed by the 1960s.
On older automatic transmissions, the process of determining and activating gear shifts is a purely mechanical process. The automatic transmission is a beautiful and complex system, with a number of components: Planetary Gearset– a collection of gears that can produce a wide array of gear ratios. Torque Converter– acts like a clutch, allows engine and transmission to be disengaged from one another Governor and Modulator or Throttle Cable– monitor speed and throttle to determine when to shift Valves– use input from the Governor, Modulator, and stick shift to control gear shifts Clutches and Bands– change gear ratios in the planetary gear set Seals and Gaskets– keep oil pressurized and contained in the system Hydraulic System and Pump– provide necessary lubrication; activate valves, torque converter, clutches and bands, and other key parts. Computer (newer cars)- takes the place of a range of devices, including valves, governor, modulator, etc. I’ll explain how each of these parts works in greater detail below. The Planetary Gearset This is a beautiful and elegant device; it operates on fairly basic principles but achieves very complex results! Figure 1: Planetary Gearset, cross-section The sun gear sits at the center, which meshes with two or more planet gears that are all attached to the same planet carrier.
These gears then meshed with the outer ring gear. All of these gears remain meshed constantly. Locking these gears together in different combinations produces different gear ratios, i.e.
Different relationships between input speed and output speed. Let’s look at a few examples. Lets’ say that the ring gear = input and the planet carrier = output. We’ll then lock sun gear so that it remains stationary.
As the ring gear turns, it causes the planets to “walk” along sun gear. This produces an output that is slower than the input, as you have in first gear. However, say we unlock sun gear and lock it to the ring gear instead. With these two elements locked together, all of the gears will turn at the same speed, so that input speed and output speed are the same. This 1:1 ratio usually occurs in third gear.
What about reverse gear? First, lock the planet carrier in place.
Then, use the ring gear as input and the sun gear as output. The planet gears will act like the idler gear on a manual transmission, causing input and output gears to spin in different directions. These are the basic principles behind a planetary gear set. As you can see, it’s important that the different parts can be locked and unlocked, joined to one another, etc. How is this accomplished? Take a look at the diagram below: Figure 2: Planetary Gearset, side view As you can see, the ring gear is used as the input, while the planet carrier is directly connected to the output shaft.
However, notice that there are clutch packs connecting the planet carrier to the sun gear, which is connected to a drum containing the pistons that activate these clutches. These clutch packs can be used to lock the planet carrier and sun gear together, so that both spin together and the sun gear becomes, in effect, the output.
Next, notice the bands on either side of the sun gear’s drum. These can be used to lock the sun gear in place. These bands are usually made of steel and are activated by a remarkable hydraulic system, which I’ll discuss a bit later. The clutches are activated by pistons, as indicated on the diagram. Hydraulic fluid enters these pistons and activates the clutch; springs cause the clutch to release when pressure is reduced. A real transmission will use two or more planetary gearsets in combination to produce up to eight different speeds.
For example, one kind of compound planetary gearset contains one ring gear, which is always the output, but has two sun gears and two sets of planet gears. The input is transferred between the small and large sun gears, while in second gear, for example, the compound gear set behaves as two planetary gear sets, essentially, with the larger sun gear acting as a sort of second ring-gear. The mechanics of it are awesomely complex! Other Gears All of these gear shifts occur when the car is placed in “drive” or “reverse.” As you know if you drive an automatic transmission, however, there are other settings that one can select with the gear shift lever. Usually, an automatic transmission will have two “low gears.” There is a second gear option, usually marked “2” or “S,” that limits the transmission to the first two gear ratios (or, in some cars, locks it in second gear.) This can be useful when driving on ice or hilly terrain; however, remember that you can’t go too fast in these gears! There is a also a first or “low” gear option, marked “1” or “L.” Like the second gear option, this can be used in difficult driving conditions or when towing a heavy load. Parking Unlike a manual transmission, automatic transmissions also have a “park (P)” gear, in which a small pin or bolt is used to lock the drive wheels in place, preventing the car from moving.
When the lever is used to select park, a spring pushes this bolt through a notch on the transmission housing, thereby keeping the transmission—and therefore the wheels—from moving. If the bolt isn’t lined up with a notch when “P” is selected, the transmission turns slightly, until the bolt can fit through a notch.
This is why automatic transmissions sometimes roll slightly when the brake is released after parking. Essentially, a very small mechanism is used to lock up the transmission. For this reason, drivers of automatic transmissions should always use the emergency brake (usually a foot brake) in addition to placing the car in park, to avoid placing undue strain on this mechanism, particularly when parked on hills. Torque Converter Like a manual transmission, automatic transmissions also have a “neutral (N)” gear. In this gear, the engine will keep idling, but the wheels won’t turn.
This gear isn’t used as frequently on an automatic transmission as on a manual transmission, as in an automatic one can come to a stop in drive without stalling. Instead of a clutch, however, an automatic transmission uses something called a “torque converter” to connect (and disconnect) the engine and the transmission. A torque converter looks like a large donut. It’s usually around a foot in diameter and is attached to the engine’s flywheel. The torque converter is a fluid coupling, meaning fluid (oil) is used to transmit the circular motion produced by the engine to the transmission. Imagine that you have two fans: one is plugged in, while the other isn’t.
You place the fans so that they’re facing one another and turn on one of them. If you hold the blades of the fan that isn’t on, it won’t turn. However, as soon as you let go, these blades will start to move, until they come close to the same speed.
This is the basic principle behind a torque converter, which uses oil instead of air. A torque converter has three parts: pump, turbine, and stator. (See figure below) Figure 3: Torque Converter, side view The turbine provides input to the transmission, while the pump is directly connected to the converter housing, which is in turn fixed to the flywheel, so that housing and pump spin at the speed of the engine’s crankshaft. Both pump and turbine have blades or fins attached, like a fan. As the pump spins, it throws fluid outwards.
This fluid, moving in a circular motion, begins to turn the turbine. Because of the configuration of blades within the turbine, the fluid changes its direction of rotation inside the turbine. Once the fluid exits the turbine, it is sucked into the stator. The stator reverses the direction of the fluid and returns it to the pump. This prevents the reversed fluid from slowing down the pump, which would make the torque converter very inefficient.
The amount of power, or torque, transferred from engine to transmission depends on how fast the pump is turning. When the engine is turning very slowly, i.e. When the car is idling at a light or stop sign, the turbine will barely turn at all. Because so little power is being transferred to the transmission, it’s easy to keep the car still by keeping one’s foot on the brake pedal. As the pump speeds up, the turbine will slowly start to accelerate, although it will lag behind the pump for awhile. When the turbine’s speed begins to approximate the speed of the pump, then a maximum amount of torque is being transferred.
Shift Circuits If you think the torque converter is clever, you should see the system devised for activating gear shifts. On newer cars, a computer is used to activate gear shifts. However, automatic transmissions evolved long before the digital age, and older automatics are entirely mechanical. So, how does a mechanical system “know” when to shift gears? This problem is more complex than simply judging the speed of the car. If you read the entry on manual transmissions, you’ll know that lower gears give you more power, allowing you to accelerate faster and to climb steep hills. If you step on the brake pedal hard, the car will stay in a lower gear for longer, in order to provide faster acceleration.
However, if you accelerate slowly, the car will shift gears sooner. When more power is required, as on a hill, the transmission will downshift automatically. The “brain” of the transmission is a hydraulic system in which oil is routed through a complex series of metal passageways (the device looks a bit like a computer circuit.) In order to shift correctly, the transmission needs input both on how fast the car is going AND how hard the car is working.
The first piece of information comes from the governor. The governor is connected to the transmission output, which in turn determines the speed of the car.
As the transmission turns, so does the governor. The governor contains a spring loaded valve and is connected to the hydraulic system.
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As the governor spins faster, the valve opens more, allowing a greater amount of oil through. The second piece of information—how hard the engine is working—comes from either a throttle valve or a vacuum modulator. In cars with a throttle valve, a cable connects the valve to the accelerator; the more the accelerator is depressed, the more the valve opens. A vacuum modulator achieves a similar effect. Both of these elements then connect to a shift circuit (See diagram below.) Figure 4: A basic shift circuit Shift valves provide pressurized oil to the clutches and bands that activate different gears by locking and unlocking components of the planetary gear set. Each shift valve controls one particular shift, i.e.
From first to second or second to third. Oil enters each shift valve in three directions: from the governor, from the throttle valve, and from the pump. When open, oil flows from the pump to the clutches and bands, causing them to activate. As the car speeds up, the amount of pressure on the right side of the valve builds, as the valve in the governor opens further. When the car is moving fast enough, the shift valve will move to the left, causing a shift to the next higher gear. However, the throttle valve also provides input into this system. If the car is accelerating quickly, the throttle pressure will be higher, counteracting the pressure from the governor.
This means that the car has to be moving faster in order for the shift to occur. The reverse occurs when you accelerate slowly.
The operation of each shift valve is triggered by the amount of pressure coming from the governor, so that particular ranges of pressure correspond to the operation of the first-second valve, second-third valve, etc. These complex interchanges are controlled by the valve body, a piece of metal with passageways molded into it, like a computer circuit. These passageways channel fluid to the appropriate valves.
The manual valve is a kind of “master valve” connected to the shift lever. When different gears are engaged, the manual valve feeds the appropriate circuits. For example, if you shift into “2,” the manual valve will feed shift circuits for the first two gears but inhibit the others. On computer-controlled transmissions, electric solenoids are used to direct fluid to the appropriate valves.
Hydraulics As I’ve mentioned above, an automatic transmission relies on a complex and extensive hydraulic system. In fact, an average automatic transmission will contain up to ten quarts of oil! Pressurized oil is used to lubricate the moving parts in the transmission, power the bands and clutches, activate shifts, power the torque converter, and cool the entire system. For this last purpose, the oil cycles through a chamber submerged in the antifreeze, in order to remove excess heat.
The automatic transmission’s pump also plays a crucial role in making sure that all parts are supplied with the necessary pressurized fluid. All in all, automatic transmissions are a mechanical marvel! To read more on a broad range of subjects from to Visit our state-specific sites for more information about online defensive driving in, and.
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Laycock de Normanville 'J type' Overdrive Unit. Overdrive is a term used to describe the operation of an at sustained speed with reduced engine (RPM), leading to better fuel consumption, 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 between engine and wheels, such that the car is over-geared, and cannot reach its potential top speed, i.e.
The car could travel faster if it were in a lower gear, with the engine turning at higher RPM. The purpose of such a gear may not be immediately obvious. The produced by an engine increases with the engine's RPM to a maximum, then falls away.
The is somewhat lower than the absolute maximum RPM to which the engine is limited, the ' RPM. A car's speed is limited by the power required to drive it against air resistance, which increases with speed. At the maximum possible speed, the engine is running at its point of maximum power, or power peak, and the car is traveling at the speed where air resistance equals that maximum power. There is therefore one specific gear ratio at which the car can achieve its maximum speed: the one that matches that engine speed with that travel speed. At travel 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 one gearing to reach maximum speed but another to reach maximum fuel efficiency at a lower speed.
With the early development of cars and the almost universal layout, the (i.e. ) ratio for fast cars was chosen to give the ratio for maximum speed. The 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 engine. The linking gearbox and rear axle is thus overdriven, and a transmission 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 optional 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, the use of 5-speed gearboxes became more common in mass-market cars. These had a direct (1:1) fourth gear with an overdrive 5th gear, replacing the need for the separate overdrive gearbox. With the popularity of cars, the separate gearbox and final drive have merged into a single. 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. Contents.
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: and. 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 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. 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, it is simply the tyre circumference multiplied by the RPM. As the tire RPM at maximum speed is not the same as the engine RPM at that power, a is used with a gear ratio to convert one to the other. 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. 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 grew, especially after the, the need for a 'cruising gear' became more pressing. 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, the transmission system normally contained two sections, the 'gearbox' or 'transmission' mounted behind the engine, and the 'final drive' mounted in the 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 to run at lower torque, by using higher RPM. As power is the product of RPM and, 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 it 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, 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' isn't applicable.
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. Overdrive should usually be selected when the average speed is above 70 km/h (40-45 mph). Dashboard indicator for overdrive (automatic vehicle, manufactured 2000) 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 next highest 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 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 having an 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. Overdrive button on the gear stick of an automatic vehicle manufactured in 2000. An overdrive consists of an electrically or hydraulically operated train bolted behind the unit. It can either couple the input directly to the output shaft (or ) (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. 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. Newer vehicles have electronic overdrive in which the computer automatically adjusts to the conditions of power need and load. In Europe The vast majority of overdrives in cars were invented and developed by a man called de Normanville and manufactured by an company called (later GKN Laycock), at its Little London Road site in Sheffield. The system was devised by, and made by Laycock through a chance meeting with a Laycock Products Engineer.
De Normanville overdrives were found in vehicles manufactured by, who were first, followed by, and to name only a few. Another British company, the former aircraft builder, built a successful all-mechanical unit for the, 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, which was fitted to many sports cars during the 1950s, and into the late 1960s.
Several famous marques used A-type overdrives, including Jaguar, Aston Martin, Ferrari, Austin-Healey, Jensen, Bristol, AC, Armstrong Siddeley and Triumph's Vanguard-powered TR sports car range, until the end of TR5 & TR250 production in 1969. In 1959, the Laycock Engineering Company introduced the D-type overdrive, which was fitted to a variety of motor cars including and, and, and also 1962-1967 (those with 3-synchro transmissions). From 1967 the LH-type overdrive was introduced, and this featured in a variety of models, including 1968–1980, the, the, early, TVRs, and. The J-type overdrive was introduced in the late 1960s, and was adapted to fit Volvo, Triumph, Vauxhall/Opel, American Motors and Chrysler motorcars, and 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 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, 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 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% (.778). For instance, the overdrive system applied to a operates on 2nd, 3rd and top gear. When engaged, the overdrive would drop the revs from 3000 by 666 RPM, or from 3500 the drop would be 777 RPM to 2723 net. The advantages this reduced rpm had on fuel consumption was most often quite near 22% decrease during highway driving. In North America In the days before automatic transmissions were common, especially in the 1950s, many American cars were available with an overdrive option.
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, 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 of the automatic. A knob connected to a, 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 petrol 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). 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. (CAFE) legislation, 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.
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) differential 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 kilometres per hour (60 mph). References Notes. 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. 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. 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. Small Volkswagens of the 1980s, such as the, 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'.