What Is Overdrive, And Why Don’t New Cars Have It?
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.
Comments
Here come the Fast & Furious jokes.
Vin Diesel intensifies
Engineering Explained lol
[DELETED]
21st!
il lead myself out
Is it bad I want to put an overdrive in a 4 speed manual that never came with it? I want to retain the 4 speed pattern
there’s nothing like a 6-speed tractive sequential gearbox.. forget about overdrives :P
So basically you want an eighties 5 speed that can be found in most light duty pickups.Nothing wrong with that, save you on some maintenance costs really.
Nope, wanting to see what you can do, and think “if I do x, then y should happen” is good. It’s people like you, people who do stuff, who gave us the internal combustion engine that we all love.
Put a 5 or 6 speed in it and use a 4 speed knob. Problem solved
Well, your options are pretty limited.
1) You could swap your differential gears to a lower ratio to lower engine speeds, but this would affect all gears and reduce usable torque
2) Slap a GearVendors under/overdrive behind your 4-speed (I know they make them for Muncie M20-M23s) which is a switchable OD that is nearly indestructible and can be used in any gear and switched manually or using automatic mode.
Simplest way to explain it is :
Overdrive makes your car shift to 3rd gear and it allows you to rev the engine higher thus means moar speed and power. Cmiiw.
It says in the article that it reduces engine speed relative to the ground speed to improve fuel economy.
That is not the function of overdrive at all….. yes an over-driven gear set can produce higher top speeds, but that is not what an overdrive (be it a separate unit or inside the gear box) is for.
My Boxster S has the 7 speed PDK box.
In truth it’s only 6 gears and the 7th effectively being an overdrive.
In simple words, overdrive gear means it’s not used for reaching top speed?
the OD off switch on my ‘90 Jeep Cherokee simply said “Power” with a Gearbox Symbol on it… pretty hilarious if you think about it seeing that it had a 4.0L straight Six with 172HP and toped out at 150kph )90mph).
Are the aerodynamics really that bad lmao
Weird fact: the automatic 42RLE in Jeep Wranglers from 2003-2011 has overdrive
not only that, it is a stupid high gear ratio at .69:1
I have a 2013 Mazda 2 and it has a overdrive button