Taken completely out of context, ‘overdrive’ sounds pretty cool. At the press of a button or a flick of a switch, it may seem like a hidden surge of power waiting at your fingertips – like you’re in the Millennium Falcon and about to unleash hyperdrive.
The reality though, is dramatically less exciting – and there’s a good reason you won’t see overdrive on any modern cars. So what is it, exactly?
What is overdrive?
Overdrive, simply put, is used as a way of improving fuel economy and reducing engine noise once a car is up to speed. That’s significantly less exciting than IRL hyperdrive.
Back in the day, top gear in most cars meant the input rotation speed from the engine was the same as the output shaft from the transmission, or a 1:1 ratio. That would leave engines sitting at high revs when at a constant speed. Keep in mind this was at a time when anything more than a four-speed gearbox was a rarity.
Rather than developing more expensive gearboxes with more gears built within, it was easier and cheaper to make an additional gearing unit on the back of the gearbox before the propshaft. And thus, ‘overdrive’ was born.
This, in essence, increases the ratio of the tallest gear. The output shaft has further to rotate, reducing the revs of an engine once at speed.
When should you use overdrive?
If you happen to own a car with overdrive, it’s not something you’ll really want to use other than when you’re at a constant speed. With gear ratios being lengthened, an engine’s peak power and torque become less exploitable and therefore acceleration is reduced.
Why don’t modern cars use overdrive?
As it often does, technology advanced far enough to make the idea of an ‘overdrive’ unit obsolete. As five- and six-speed gearboxes became more commonplace, the need for an overdrive dwindled.
Now with automatic gearboxes offering as many as 10 speeds in the case of some Fords, it’s effectively built into most gearboxes.
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