What does "overdrive" in an automatic transmission do?

[atom]

People tell me overdrive is basically a gas saving gear. But isn't that what 4th gear is for in a 4 speed transmission?
So doesn't that make turning off overdrive basically the same thing as putting the car into "D3"?

 

[toph99]

it's basically and extra gear i believe. and yes, it does save gas

 

[squirrel dog]

Thats correct,although the od in an auto trans is most times at a higher gear ratio due to gear slippage.The fourth gear in a manual might be at a 3.10 gear ratio,an auto trans might be at a 2.70.

 

[mztykal]

oh kewl, i always wondered what that lil button on the shifter was...

 

[Handle]

You can also use it if you're having difficulty climbing a hill at relatively high speeds on a car with a weak engine and high drag coefficient (such as an old Tercel with a huge "Student Driver" sign on the top).

Edit: Sorry, that is you disengage it if having difficulty with hills.

 

[Viperoni]

Lol, the mis-informed public.

Sometimes in Manual transmissions, are gear with a ratio lower than 1:1 (IE, engine spins @ 100rpm, transmission output shaft spins @ 99 or less rpm)

But in automatics, all the overdrive button does is lock up the torque convertor (the thing the fluidly couple's your engine to the transmission)
Normally they are quite efficient, but when locked up, they are 100% efficient.

The purpose of the torque convertor is very similar to a clutch in manuals, but it engages/disengages for you

Disengaging overdrive just lets the torque convertor act its usual way, only needed if you have a leadfoot or towing heavy stuff.

Example:

4th gear = 2000 engine rpm
4th gear + OVERDRIVE = 1750 engine rpm

That's about it folks!

 

[Viperoni]

How you can tell when overdrive is working:

Drive at a constant speed (45mph)
Slowly keep pushing the gas until you feel a faint "shift"
That's when the torque convertor "unlocks"

If you keep pushing, you can get it into third.
Hopefully, you won't need to push it into second :Q

 

[atom]

Thanks, I'm sort of getting it now (still confused a bit though).

So during what situations would you want to disengage overdrive?

 

[Tominator]

There are many designs called 'overdrive.' Some just lock the torque convertor while other designs have a higher gear. It might be a 4 speed with 2 ratios very close together.
The higher gear would be the OD.

My Mitsubishi van has the forth gear. The owners manual suggests turning the OD 'off' when going down a hill, to add to the braking, or up a steep hill.

It IS 'basically' there to save gas.

 

[Viperoni]

When you have a big load, when you need to accelerate quickly at speed (30+mph), that's about it.

2 ways it affects performance.

1. By locking up, it lowers your rpm (increases has mileage)
2. Takes longer to accelerate because the torque convertor has to unlock (to rev into the power band )

The torque convertor basically works like a clutch, but grips more at higher rpm.


Scenario:

Imagine driving a manual tranny car.
you're doing 30mph in 5th gear.
You step on the gas.
Not much happens, right?
You shift into 4th, it picks up a bit better.

That's like the difference between Locked and unlocked


Another example

700rpm (idle) Very LITTLE grip.
900rpm SOME grip.
1200rpm More grip.
1500rpm MAXIMUM grip

As the rpm's go up, the grip stays the same.

Note: The torque convertor uses the tranny fluid to transfer the engine's power, so it can't be burned out....but the auto tranny has many other smaller clutch's/gears/actuators inside of it that can break (that's why doing neutral drops is BAD in an auto tranny car)

 

[Demon-Xanth]

Note: OD in some cars indicates the gear above a 1:1 ratio, in others it locks the converter. Some cars it does both. In our Jeep, OD is most definately 4th gear and 3rd isn't even stated as "D". Same with my 1991 Grand Prix

 

[DABANSHEE]

Tom is right there are many definitions of 'overdrive'

By some standard any ratio under 1:1 is termed an overdrive as 1:1 is often referred to as drive.

In many manuals & some automatics, 4th is the drive gear (arround 1:1), while 5th is an overdrive (under 1:1), however some cars have sports style close ratio 5 speeds where 5th is the drive gear & is not an overdrive.

In many automatics, overdrive is just when the Torque converter locks in, whether manualy or via a button. Some Automatics actually have an extra overdrive gear, in which the torque converter lock up in.

Some cars have dual range selectors at the back of the gearbox or in the differential that works like low range in a 4x4 but in reverse, as in it gives you a extra high range. Either they just work on just the top 2 gears or on all the gears. Many British cars from the 60's have such as setup. I remember driving a 4 speed manual MKII Jag & every time I turned a corner it seemed to change gears like an automatic, well what was happening is that it had a electronic overdrive setup controled by a column stalk, I was flicking it by mixtake thinking it was the indicator stalk. Many Triumph Cars also have such as setup to. Actually some English cars as early as the 50's & as late as 1980 (it was an option on the TR8) had this type of overdrive.

Some automatics just have a switch on the gearlever, that gives you the choice of 'highway' or 'sports' mode. When in 'highway'mode it changes up earlier in the rev band & when in 'sports' mode it changes up later in the rev band, but many marketing/sales types will referr to it as a overdrive.

 

[Soybomb]

You guys know that not all torque convertors lock up for overdrive right? Not all torque convertors even lock up.......
Tom's right, many definations