Download file Free Book PDF Land Rover Overdrive Manual at Complete Installation Manual Series Land Rovers Options Fairey Overdrive. OVERDRIVE UNIT AND SELECTOR MECHANISM. -. Ye |. 17 Gasket, overdrive to transfer box. Dipstick FITTING INSTRUCTIONS. (in). Fairey Overdrive Fitting Instructions. Page history last edited by Stephen Jarman 10 years ago. Download: Fairey Overdrive Fitting

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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. When less load is present, it manuaal back to OD.

As the engine requires more power to overcome internal friction at higher RPM, this means more fuel is used simply to keep overdriev engine running at this speed. Borg Warner provided the box that was factory-installed between the transmission and a foreshortened driveshaft. In these cases, there is no separately identifiable “overdrive” unit.

American cars with large-slow-revving engines would use higher ratios, European compact cars with small high-revving engines were lower.

Overdrive (mechanics)

In the days before automatic transmissions were common, especially in the s, many rear-wheel drive American cars were available with an overdrive option. Generally speaking, overdrive is the highest gear in the transmission. The final drive then took this output and adjusted it in a fixed-ratio transmission overdrife that was much simpler to build.

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.

Land Rover Restoration, Tech, Maintenance and Repair – O

Clearly this condition calls for a different gear ratio. Indeed, in modern vehicles this is common. These produce two primary forces slowing the car: Thus the output shaft may be “overdriven” relative to the input shaft. For other uses, see Overdrive. As noted earlier, however, this would cause the engine to operate at too high an RPM for efficient cruising.


The Gear Vendors U. Such add-on overdrive boxes were available from the s to the s for cars and light trucks.

This allows the vehicle to achieve better fuel efficiency, and often quieter operation on the highway. This is known as the point of maximum power. Newer vehicles have electronic overdrive in which the computer automatically adjusts to the conditions of power need and load. The obvious solution to this problem would be to add more gears to the transmission.

However, due to historical particularities, this was not always practical. The Volvo version kept the same package size as the J-type but with the updated 18 element freewheel and stronger splines through the planet carrier.

The former varies roughly with the speed of the vehicle, while the latter varies with the square of the speed. The impetus is to minimize overdrive use and provide a higher ratio first gear, which means more manuap between the first and the last to keep the engine at its most efficient speed. When it is off, the automatic transmission shifting is limited to the lower gears. Please help improve this article by introducing citations to additional sources.

Calculating these from first principles is generally difficult due to a variety faireey real-world factors, so this is often measured directly in wind tunnels and similar systems.

As popular cars became faster relative to legal limits and fuel costs became more important, particularly after the oil crisisthe use of 5-speed gearboxes became more common in mass-market cars.

Another British company, the former aircraft builder Faireybuilt a successful all-mechanical unit for the Land Roverwhich is still in production in America today. In practice this gives the driver more ratios which are closer together providing greater flexibility particularly in performance cars. At the maximum possible speed, the engine is running at its point of maximum power, or power peakand the car is traveling at the speed where air resistance equals that maximum power.


There is no longer a drive shaft between them and so the notion of “direct drive” isn’t applicable. This defines the maximum speed the vehicle is able to reach. 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. Since the overdrive function, if enabled, could be shifted by simply easing up on the accelerator without depressing the clutch pedalthe action was much like a semi-automatic.

It may also be advantageous to switch it off if engine braking is desired, for example when driving downhill.

The purpose of such a gear may not be immediately obvious. 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.

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.