This fitting arrangement (Fig 2) borrows something from the principle of the three legged stool; it is inherently stable and compact and the mounting points do not run away from their responsibilities by overloading one of their fellows.

Although this was chosen as the most suitable configuration, a thorough understanding of the interaction of the components and some precise machining is required to ensure the assembly of a unit which is smooth and reliable. A conventional prop. shaft then runs from the rear of the overdrive to the rear axle of the car. To allow the speedometer to work correctly in all gears including overdrive, the speedometer drive at the rear of the unit is used, and attached to the existing cable via an adaptor box.

The adaptor box is calibrated to also take into account any variations in tyre size or axle ratio and can allow for any calibration drift which may have crept into the speedometer instrument over time.

The main criteria for fitting an overdrive are that there is sufficient room under the car, and that there is a minimum of 38" of prop. shaft. Having said that, development is under way for a system for use on the Torque tube Silver Ghost and Phantom I cars. The same principle is used, but with the overdrive mounted inside a "Torque Box" which replaces the front half of the Torque tube.

It is a policy to avoid, as far as possible, altering the vehicle components, so allowing the car to be easily returned to original condition with no evidence that it had been "tampered with". With the 3 1/2 and 4 1/4 Derby models it is necessary to drill a hole for mounting an interlock switch near the gear lever, but this would never be done without the approval of the customer. Certainly cutting prop. shafts, welding brackets onto the chassis and modifying the bodywork are definitely not considered to be options. Only high quality components are used and, as far as possible, parts are nickel plated or powder coat painted.