Friday, July 06, 2018

The Laycock de Normanville Overdrive

Invented by Edgar de Normanville, and manufactured by automotive supplier Laycock Products, the unit basically consisted of a solenoid-activated planetary gearset residing between the standard manual transmission and driveshaft, offering a reduction in gear ratio at the driver’s command.

At the press of a button or flip of a switch (depending on the car) in the cockpit, the overdrive would engage and lower the engine speed relative to the driveshaft, markedly improving fuel economy.

Advantages? Transmission development has always been pricey, and as fuel economy began to become a priority for automakers, the idea of a fuel-saving external add-on to an existing 3- or 4-speed manual transmission was an appealing one. Not only that, but the nature of the Laycock Overdrive meant that it could be engaged at any time, even in the lower gears, effectively doubling the number of ratios at a driver’s disposal. A 4-speed tranny became an 8-speed, for instance.

As all-in-one 5-speeds with integrated overdrive became de rigeur, the Laycock Overdrive faded from the scene.

The factory installed the Volvo M46 4-speed + overdrive transmission on Volvo 240s and later 740s

Until the late ’80s, the Laycock Overdrive was fitted to a wide variety of cars, not just Volvos: Jaguars, MGs, Austin Healeys, Alpines and Triumphs, among others, were available with the unit.


  1. Dad's account on how he shoehorned a M41 transmission with an OD in to his 62 PV544 Volvo.

  2. The "A" Type O/D in my 1969 Triumph TR6 works on 2nd, 3rd, and 4th gears....effectively giving me a 7 speed transmission. The later TR6's went to a "J" Type O/D unit that only worked on 3rd and 4th gear.

    I have found on certain twisty roads that I can leave the lever in 2nd gear and flick the O/D off for the corner and then on again for the straights.

    Makes for some fast and smooth motoring...