Whilst having superb off-road capability, a Series Land Rover's on-road performance is considered less than satisfactory by many owners. This is particularly true regarding fuel economy on long journeys. Thus, the Fairey overdrive was offered as an optional extra on many Series III Land Rovers before the vehicles left the factory. They could also be retro-fitted to earlier models by dealers, or owners themselves, at a later date. When the Fairey overdrive ceased to be manufactured, parts became difficult to obtain but still a thriving market developed for used units and that continues to this day.
The overdrive enables 21.8% lower engine revs for the same road speed. If the same driving style is continued then the driver experiences a significant increase in fuel economy. A Series Land Rover with a standard 2286cc or 6-cylinder engine has trouble keeping up with modern traffic. So, a further advantage to having an overdrive fitted is that it is easier to maintain the higher roadspeeds demanded on todays roads.
A steady speed of 55mph can be maintained at 2700rpm with the 2286cc engine and 7.50-16 tyres. This is quite satisfactory performance for a Series Land Rover.
Lower engine revs are particularly beneficial for the pre-1980 2286cc petrol and diesel engines that have the 3-bearing crankshaft, as these tend to flex at high revs and are known to bend and wear out the main bearings more rapidly. Infact the pre-1980 diesel crankshaft has been known to break under high revs.
Series Land Rover owners have commented that they have not noticed the 2-3mpg fuel ecomony that others have reported. This may be due to the fact that the original Fairey overdrive is high geared and it is easy to get into the habit of letting the engine labour in a higher gear than necessary. The overdrive ceases to be economical when used at full throttle opening. This is particularly the case when used in conjunction with the standard Zenith 36IV carburettor. The moral of the story is to use the overdrive to reduce revs but not to the extent that you labour the engine and avoid using full throttle as much as possible.
A problem with the Fairey overdrive is that it uses its own oil supply for lubrication and this is only a few hundred cc capacity. Given the generally leaky condition of most things associated with Series Land Rovers, it is perhaps not surprising that the overdrive is prone to wear due to lack of lubrication. It is important to check the overdrive's oil level dipstick regularly and keep topping up with EP90 oil as required.
Fortunately Fairey overdrives are now being remanufactured and all parts are available to the original specifications, so old units can be rebuilt if required.
Fitting an overdrive should atke no more than 3 hours
and is well within the capabilities of the average DIY Series Land Rover owner.
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