In simple terms, overdrive is an extra gearbox behind the vehicle’s main gearbox, giving direct drive when not engaged or a higher ratio when engaged. It can provide an extra-high top gear for relaxed cruising, reducing engine revs and wear at speed, improving fuel economy and the life of the engine. Overdrive is generally confined to rear-wheel-drive cars, where there is adequate space for the overdrive unit to be incorporated.

For decades, manufacturers were constrained by needing a very low first gear for steep and rough unmade roads, and limited by technology and cost to have relatively few gears in the gearbox, close enough in ratio to stay within the engine’s effective power band. This necessitated a lower axle ratio than might otherwise be desirable for high-speed cruising. Motoring journalists and the motoring public also favoured great flexibility in top gear.

Overdrive was the perfect solution: cars could have low overall gearing for good acceleration and hillclimbing but, with a flick of a switch, an extra gear could be engaged to give all the benefits of high gearing.

Though it only came into common use after WW2, overdrive was first offered on the 1914 Cadillac range, which boasted electrically selected final drive ratios of 4.04 or 2.5: 1, offering effectively two sets of gears, high and low ratio. The Cadillac overdrive was part of the rear axle and dual-ratio axles became quite popular on expensive, heavy cars for a while but, as engineers discovered the importance of minimizing unsprung weight with live axles, overdrive migrated to the back of the gearbox.
Borg Warner made the overdrives often fitted to American cars in the 1950s. The usual method of engagement was via a switch under the accelerator pedal; by lifting briefly in top gear, overdrive would be engaged, then it could be disengaged by accelerating hard. This form of ‘automatic overdrive’ was first seen on 1934 Chrysler and De Soto Airflow models.

The vast majority of overdrives fitted to British cars were Laycock de Normanville units (though the biggest user of these was Volvo, who fitted over a million of the 3.5 million made during 40 years of production). British inventor Captain Edgar J de Normanville became technical editor of The Motor in 1908 and later technical director of Auto Transmissions. In 1935 Humber adopted his de Normanville Safety Gear, but it was post-war that his solenoid-operated epicyclic overdrive entered production by Laycock, later GKN.

A typical overdrive unit contains a sun wheel fixed to a sliding cone clutch, which is lined on both sides (see illustrations above and on the facing page). Around the sun wheel and inside an internally toothed annulus runs a planetary gear set, whose carrier is fixed to the input shaft. The annulus is fixed to the output shaft.

When overdrive is not engaged, the cone clutch grips the annulus and so locks sun wheel, planetary gears and annulus together. That effectively gives direct drive to the output shaft.

Engaging overdrive makes the clutch disengage and lock against the outer casing, which stops the sun wheel from turning. Now the input shaft turns the planet carrier around the sun wheel, driving the annulus and turning the output shaft slightly faster than the input. The ratio of input to output is usually in the order of 0.8: 1.

The first car to feature a Laycock de Normanville overdrive was the Standard Vanguard of 1948; for the Triumph TR from 1955 it was set up (via detents on top of the gearbox) to operate on second, third and top gears, so that it effectively gave the car a seven-speed gearbox.  

The A-type overdrive was also used by Jaguar, Aston Martin, Ferrari, Austin-Healey, AC, Armstrong Siddeley, Bristol, Jensen and more. In 1959 Laycock introduced the D-type, whose most prolific users were Volvo, Sunbeam, Triumph and MG. The LH-type of 1967 was used on MGB, Ford Zephyr, Reliant Scimitar, Gilbern and TVR.

A few years later it was joined by the J-type, which appeared in Volvos, Triumphs, Vauxhall/Opels, Fords, AMCs and Chryslers. The final development was the P-type, offered in Volvo form or with a larger output shaft for US applications, marketed by Gear Vendors Inc of California, who would go on to purchase all the overdrive assets of GKN plc in a million deal in February 2008. Gear Vendors aims to maintain supplies for millions of cars fitted with overdrive from the 1920s on. It even makes a kit to fit overdrive to a Shelby Mustang for 95.

Manufacturers tended to use a lowish axle ratio when overdrive was an option so that, if overdrive is not fitted, the cars are tiring on long runs. Conversion is the answer; if you cannot find an overdrive gearbox, switchgear and ancillaries from a scrap vehicle, try the specialists. For example, Octarine Services supplies a complete conversion kit for an MGB for £495 plus VAT, to which you must add fitting costs, which are likely to be almost as much again. Conversion kits for several marques are available (O/D Spares do them for Aston Martin and Rolls-Royce).

The best-known overdrive specialist in the UK is Overdrive Repair Services of Sheffield, run by ex-Laycock employees with a good stock of Laycock parts, machinery and tooling. As well as rebuilt exchange overdrive units and conversions for Daimler SP250 and Bristol 400, there is even a Mitsubishi-approved conversion for the L200 Diesel Pick-up for £1435 plus VAT. Joe Winter’s company can also supply an independent sealed-unit overdrive for fitting to any rear-wheel-drive vehicle, given suitable mounting points, for £695 plus VAT.

The Laycock overdrive operates at very high oil pressures – c500psi – and requires specialist tools to rebuild, but some checks can be carried out on overdrives that fail to function. First, check the oil level (usually shared by gearbox and overdrive) and top up if necessary. Modern transmission oils are not all suitable: check the spec and, if in doubt, consult the manufacturer. Anti-friction additives must be avoided. Classic oil makers’ websites specify which are suitable for overdrives.

If this makes no difference, check that the solenoid is operating: with the ignition on, engage top gear and switch on overdrive – you should hear a click from the solenoid. If there is no click, try running a live wire direct to the solenoid – the fault may be in the relay, the detent switches or the wiring (don’t forget to check the fuse). If it still doesn’t work, remove and clean the solenoid; if that fails, replace it.
If the solenoid is operating but overdrive does not engage, check the setting of the operating arm. Your car workshop manual will state the correct setting.

If overdrive still doesn’t work, try changing the gearbox and overdrive oil, including dropping the overdrive pan and cleaning the magnet and filter therein with paraffin. If you have the required tools, remove the oil pressure relief valve assembly, clean and reassemble. The next check is of oil pressure (21psi disengaged, 380-570 engaged, depending on model, no more than 3sec to drop to 21 when disengaged). Beyond this, it’s time for expert attention.

Other overdrive types were used in the UK; best known is the Fairey/Superwinch overdrive fitted to Land-Rovers, a two-speed mechanical gearbox with its own gearlever. Toro and Roverdrive made similar units. In the 1950s you could buy the Murray mechanical overdrive gearbox for Ford 100Es, or the HandA overdrive, which featured an epicyclic gearbox similar to the de Normanville but with a vacuum solenoid and which was offered for most 1950s Fords, Vauxhalls and BMCs.