What is overdrive on a car?

What is overdrive on a car?

Overdrive in a vehicle is "an automotive transmission gear that transmits to the drive shaft a speed greater than engine speed”.

That strict definition comes courtesy of the Merriam-Webster dictionary. But more broadly, what does overdrive mean? What is over drive? What does overdrive do? And how do you turn overdrive on? So many questions and all (most?) of them valid, so here goes…

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So, what does overdrive mean in cars? In layman’s terms, overdrive is any gear with a ratio greater than 1:1. The higher the gear, the lower the ratio and when the powertrain reaches those lower gear ratios it allows the engine to perform at lower RPMs, usually during open-road cruising.

For example, a Toyota LandCruiser 300 Series with a 3.3L V6 twin-turbo diesel engine and 10-speed automatic transmission has transmission ratios of - 1st 4.923 (number of times the input shaft turns to rotate the output shaft once), 2nd 3.257, 3rd 2.349, 4th 1.944, 5th 1.532, 6th 1.193, 7th 1.000, 8th 0.801, 9th 0.661, 10th 0.613.

Although factors like the final drive (differential) ratio and the rolling radius of your car's tyres can impact the equation, in general terms those last three ratios are 'overdriven' with 10th being the final overdrive gear.

Overdrive is all about the ratios.

Overdrive means engine RPMs are lower during open-road cruising, thus improving fuel efficiency, lowering noise, vibration and harshness (NVH) levels and reducing wear and tear on your vehicle’s powertrain.

On some vehicles, most of them older models, overdrive is activated via a button but nowadays this function is engineered into the car’s powertrain and it operates without your input.

In a manual vehicle you’re in charge of gear selection, so you shift up to the highest gear possible to make the most of overdrive ratios; with regards to overdrive in an automatic car, the smarter-than-you transmission does the work for you.

Suzuki Ignis interior. (Image: Justin Hilliard)

What is the function of overdrive in a car? Haven’t you been concentrating? It’s to lower fuel consumption and reduce strain on the engine at high speeds and then the follow-up question – What is the benefit of putting a car in overdrive? – pretty much answers itself because the welcome side effects of overdrive include lowering NVH levels and minimising powertrain wear and tear because the engine is not working as hard.

So, now you’ll likely be wondering: Should overdrive be on or off? If you’re in a vehicle which gives you the option to switch it on or off, sound advice is to keep it on unless you’re towing or low-range 4WDing because in those situations you need high revs and the ability to tap into plenty of power and torque. Overdrive is beyond the vehicle’s peak power point so it’s for highway driving or the like.

When should you use overdrive? When you’re on a highway or otherwise open-road driving where you’re doing consistently high, but legal, speeds.

Overdrive improves fuel efficiency.

How do you use overdrive? Switch the overdrive button on or off (o/d off) or, more likely, just select the highest gear in your conveyance of choice – or let the auto transmission do it for you – and enjoy the smooth, quiet ride or at least some approximation of that.

Where can you find out more information about overdrive in cars? Ask your opinionated uncle Tarquin about overdrive at the next family barbecue – he knows everything – or better yet check your vehicle’s owner’s manual.