The GT6 has aways been offered with an optional Laycock de Normanville D-Type overdrive. Overdrive is an additional 'epicyclic' gearbox bolted at the back of the normal gearbox (necessitating a shorter propshaft). It is actuated by a solenoid. It reduces the overall gearing by 20% on 3rd and 4th gears.

Captain Edgar Joseph de Normanville invented the epicyclic overdrive manufactured by Laycock Engineering of Sheffield, hence the name "Laycock de Normanville" (it is rumoured they had a chance meeting in a pub). It is characterised by its ability to instantaneously switch from 'overdrive' to 'non-overdrive' at the flick of a switch, initially a lever on the steering column, and on the Mk3 a switch on the gear lever. The first Triumph to feature overdrive was the Standard Vanguard of 1948 which was fitted with what is know as the A-Type. Other manufacturers soon followed suit. The different types of overdrive are listed in the table below, with some of the manufacturers who used them. The P-Type was an improved version of the J-Type.

Type Introduced Manufacturer Use
A 1948 Jaguar; Aston-Martin; Ferrari; Austin-Healey;
Jensen; Bristol; AC; Armstrong-Siddeley;
Triumph TR
Cars 2 Litres +
D 1959
Volvo 120 & 1800; Sunbeam Alpine & Rapier; Triumph Spitfire & GT6; MGB to 1967
"Smaller Cars"
LH 1967
MGB 1968 on; MGC; Ford Zephyr; Triumph;
Reliant Scimitar; TVR; Gilbern
As D-Type
but stronger
J 1972 Volvo; Triumph; Vauxhall/Opel;
American Motors; Chrysler; Ford Transit
As D-Type
but stronger
P -

Volvo; Gear Vendors (USA)

As J-Type
but stronger

The gear reduction ratio of the overdrive varies from 20% to 28%  with different cars. To determine the percentage locate the serial number plate attached to the unit. This should be in the form of two numbers followed by a forward slash and two sets of six figure numbers divided by another forward slash. The first two numbers refer to the percentage reduction ratio. In the example below the first two numbers are 28 which gives a 28% reduction. In top gear, which normally is 1:1, this would become 0.72:1. In my particular GT6 this number is 25 so the ratio in top would be 0.75:1.

Below are images of the different types of overdrive.



The J-Type overdrive was never fitted to the GT6, but was fitted to the Spitfire 1500,  all Dolomites, later 2000/2500, and late TR6s and the Stag (the earler LH-Type overdrive was, according to Graham Robson, fitted to the Spitfire Mk4). It is distinguished by the solenoid being on the LHS (all A and D types have the solenoid on the RHS). The J-Type Overdrive can been retro fitted to many earlier cars, but as they are designed to fit the single-rail gearbox some part swapping needs to be done to fit it to a 3-rail box, but it can be done! Experts on this were Tony Lindsey-Dean, John Kipping and John Kippings protégé Mike Papworth, but there may be others. Below is my particular installation of a J-Type fitted to a strengthen 3-rail gearbox. My switch is on the gear lever, unlike the standard GT6 Mk2 where is is on the steering column.

Over a period of 40 years, Laycock Engineering manufactured over three and a half million overdrive units. In 2008 the U.S. company Gear Vendors, Inc. of California purchased all the overdrive assets of GKN (who had taken over Laycock Engineering) to continue production of the U.S. version and all spares for J and P types worldwide.