Overdrive

overdrive /oh-ver-dryv/ noun (countable)

Overdrive is an automotive transmission mechanism that allows for an additional, higher gear ratio to be engaged, reducing engine speed for a given road speed to improve fuel economy and reduce noise during high-speed cruising. This was an electro-hydraulic contraption bolted to the back of a normal four-speed transmission that acted as a secret "half-gear" for effortless motorway travel. For generations of British sports and saloon car drivers, the satisfying flick of the overdrive switch and the corresponding drop in engine revs was a defining feature of relaxed, long-distance motoring. It was a charmingly eccentric piece of engineering, a crutch that became a much-loved character trait.

The Full Story of Overdrive

In the 1950s and 60s, as Britain's first motorways began to snake across the country, a problem became apparent. Most British cars were fitted with four-speed gearboxes, which were perfectly adequate for the twisty A- and B-roads they were designed for. At a constant 70 mph, however, these gearboxes left the engine buzzing away at high revs, creating a wearing drone and drinking fuel at an alarming rate. A fifth gear was desperately needed, but designing and tooling up for an entirely new transmission was a colossal expense that many manufacturers were keen to avoid.

The solution came from a Sheffield-based engineering company called Laycock de Normanville. They perfected a compact epicyclic, or planetary, gearbox unit that could be bolted directly to the back of a conventional transmission. This was the overdrive. At the flick of a switch, usually mounted on the gear knob or a steering column stalk, an electrical solenoid would engage a clutch within the unit. This would bring the planetary gearset into play, creating a higher gear ratio. The engine revs would drop by about 20%, the noise would subside, and the car would settle into a relaxed, long-legged cruise.

It was a brilliantly pragmatic, if slightly Heath Robinson-esque, solution. The system was usually configured to work on third and top gear, effectively turning a four-speed gearbox into a six-speed with two sets of ratios for different situations. A driver on a fast, flowing A-road could use third and third-overdrive for brisk progress, and fourth and fourth-overdrive on the motorway. It was a versatile and engaging system that demanded a little more thought from the driver than a simple five-speed box.

The Laycock unit became ubiquitous, a hallmark of almost every self-respecting British sports or saloon car from the 1950s to the late 1970s. It was fitted to everything from Triumph TRs and MGBs to Rover and Jaguar saloons. Its excellence was such that it was also widely used by European manufacturers, most famously by Volvo, who fitted it to their cars for many years.

Ultimately, the overdrive was a clever stopgap. By the late 1970s, Japanese and European manufacturers were arriving with cars that had slick, reliable five-speed gearboxes as standard. The separate overdrive unit, with its extra complexity and cost, began to look like a relic from a bygone era. The simple, intuitive five-speed H-pattern won the war, and the distinctive click of the overdrive lever was consigned to the classic car world.

For The Record

Who were Laycock de Normanville?

They were a British engineering company based in Sheffield that designed and manufactured the dominant type of automotive overdrive unit used by most British and many European car manufacturers from the 1940s to the 1980s.

Could you use overdrive on every gear?

Generally, no. It was usually only enabled on the higher gears, typically third and top. Engaging it in first or second gear would put too much strain on the unit's clutches, so manufacturers fitted an electrical lockout switch to prevent this.

Is it the same as an automatic gearbox's "overdrive"?

Conceptually, yes, but mechanically, no. In an automatic, "overdrive" simply refers to the highest gear having a ratio of less than 1:1, meaning the output shaft spins faster than the engine. The classic British overdrive was a separate, add-on unit to a manual gearbox.

Was it reliable?

For the most part, yes. The Laycock units were well-engineered and robust. Common problems after many years of use were usually electrical, relating to the solenoid that engaged the system, or caused by low oil pressure in the unit itself, often due to a simple clogged filter.

Why did Volvos use a British overdrive?

Because in the post-war era, the Laycock de Normanville overdrive was simply the best and most readily available off-the-shelf solution in the world for adding a high-speed cruising gear to a manual transmission. Volvo, like many other European manufacturers, found it easier to buy the proven British unit than to engineer their own.