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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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Overdrive (mechanics)

The point of maximum power is somewhat lower than the absolute maximum RPM to which the engine is limited, the ” redline ” RPM. For other uses, see Overdrive. Mmanual is known iverdrive the point of maximum power. 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.

Land Rover Restoration, Tech, Maintenance and Repair – O

The device for achieving an overdrive transmission was usually a small separate gearbox, attached to the rear of the main gearbox and controlled by overcrive own shift lever. A fxirey connected to a bowden cablesimilar to some emergency brake applications, was also provided to lock out the unit mechanically.

Borg Warner provided the box that was factory-installed between the transmission and a foreshortened driveshaft.

In practice this gives the driver more ratios which are closer together providing greater flexibility particularly in performance cars. Retrieved from ” https: Every cycle of the engine leads to wear, so keeping the engine at higher RPM is also unfavorable for engine life.

These produce two primary forces slowing the mannual Clearly this condition calls for a different gear ratio. This was particularly important in the early days of cars, as their straight-cut gears were poorly finished, noisy and inefficient. When it is off, the automatic transmission shifting is limited to the lower gears. The rotational speed of the wheels for that given forward speed is simple to calculate, it is simply the tire circumference multiplied by the RPM.


Thus the output shaft may be “overdriven” relative to the input shaft. 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.

Overdrive allows the engine to operate at a lower RPM for a given road speed.

In the U. It may also be advantageous to switch it off if engine braking is desired, for example when driving downhill. As the engine overdrve more power to overcome internal friction at higher RPM, this means more fuel is used simply to keep the engine running at this speed. 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. When engaged, the overdrive would drop the revs from by RPM, or from the drop would be RPM to net.

Fairey Overdrive Fitting Instructions

When using overdrive gearing, the car’s engine speed drops, reducing wear and normally saving fuel. The automatic transmission automatically shifts from OD to direct drive when more load is present.

As power is the product of RPM and torquerunning the shaft at higher RPM allowed more power to be transferred at lower torque. 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.

The rotation speed problem comes into effect when the differential gearing is a high ratio and an overdrive is used to compensate. 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 article relies largely or entirely on a single source. 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. In the automotive aftermarket you can also retrofit overdrive to existing early transmissions. With the use of front-wheel drive layouts, the gearbox and final drive are combined into a single transaxle.

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. When less load is present, overdtive shifts back to OD.

In this case, switching it off can help the transmission to “decide”.

Today, most petrol and diesel cars and trucks come with an overdrive transmission because of the benefit to fuel economy. 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: It is also why more than one overdrive gear is seldom seen in a vehicle except in special circumstances i.