Overdrive as a concept seems pretty awesome - normally displayed as a button or switch, it almost seems like a hidden source of power that can suddenly be unleashed with a press or a flick. Unfortunately, it’s the complete opposite, being used in older cars to produce decent fuel economy and less engine noise once up at a cruising speed.
Overdrive as a definition is simply having a faster transmission output shaft speed than the input rotation speed of the input shaft from the engine. This means torque is effectively being multiplied in a positive sense so being ‘overdriven’, creating a more effortless driving experience.
‘Underdrive’ is therefore the opposite, where the engine is spinning faster than the resultant rotation of the output shaft. This is experienced in the lower gears (normally up to third gear) which are used for acceleration before cruising gears are employed.
You may associate ‘going into overdrive’ with classic cars, particularly ones that only have four forward gears. This is because, back in the day, top gear in most cars featured a gear ratio of 1:1. This meant that at highway speeds, cars would have to sit at high engine speeds, wasting fuel and making a bit of a racket. Gear ratios are written as the input shaft speed against the output shaft’s constant of one, determining the differences in drive after the power has been sent through the transmission. So a 1:1 ratio is known as ‘direct drive’ as there is no differential in rotation across the transmission.
To solve the issue of inefficient cruising with a 1:1 ratio, engineers decided that it would be easier to make an additional gearing unit on the back of the gearbox before the propshaft. Redesigning entire transmissions with more gears would be a much more expensive and time-consuming task, so the overdrive unit was born.
An overdrive is effectively a gear-reduction system made up of a combination of sun, planetary and rings gears. When not in use, the overdrive allows direct drive to take place with the sun gear turning the ring gear. Once overdrive is enabled, the sun gear is fixed in place and the planetary gears are brought to life, rotating the ring gear.
The increase in gearing occurs due to the planetary gears having more teeth than the original sun gear, thus turning the ring gear further per rotation. This means that if the planetary gears can rotate the ring gear 1.2 times compared to just one turn for direct drive, the output shaft will have turned 20 per cent further than usual. This will allow the engine to operate at a lower RPM for a given road speed while cruising.
An overdrive system is generally actuated by using a mechanical linkage or electronic solenoid displayed as a button or lever on the gearstick or dashboard. Although it could be used as a function for every forward gear, most systems are locked until either third or fourth gear is selected to avoid lugging the engine after each gear change.
The addition of an overdrive system to a transmission began to fall away in popularity when transmission technology expanded to five- and six-speed gearboxes. The additional gears have ratios higher than 1:1, creating the overdrive feature needed for cruising and motorway driving. This is then further enhanced by the latest crop of transmissions that feature 10 or 11 forward gears, all of which can combine to produce fairly nifty fuel consumption figures.
You may not have to look too far back for automatic cars featuring overdrive, but the technology has been out of date for quite some time in manual transmissions. Saying that, an overdrive unit certainly had its place back in the day and showed one of the first ventures towards creating reasonable fuel economy on a long cruise.