Od Button

[BAnuradha]

Hi Friends

Please let me know the purpose of OD button comes with auto vehicles

Thanks

[Bugatti]

  BAnuradha said:

Hi Friends

Please let me know the purpose of OD button comes with auto vehicles

Thanks

OD = Over Drive...

If it is a 4 speed automatic, the OD button when switched "ON" changes to the 4th gear.. If it is in "OFF" position, means car will not shift beyond 3rd gear..

This is what I know. I leave it to the experts to comment in detail.

[mirage7777]

I am not an expert my self -- but i think OD is to change to the fifth gear from the 4th gear. If OD -OFF -- then it will go upto the 4th only.

[Parak]

Hi

Bugatti and Mirage7777 simply explained the OD giving examples of 4 speed and 5 speed gear boxes. Actually OD stands for Over drive. Means in Over drive mode (in the highest gear could be 4th or 5th or any number) wheels spins faster than the engine (in technical terms Wheel RPM > engine RPM). Normally old days with 4 speed gear boxes when the car is in top gear(4th) engine revolving RPM and wheel revolving RPM used to be the same. Correct me if I am wrong

Cheers

[ucscsades]

I believe this has been discussed in detail ages ago? Check forum archives...or www.google.com , great site!

[harshansenadhir]

  Parak said:

Hi

Bugatti and Mirage7777 simply explained the OD giving examples of 4 speed and 5 speed gear boxes. Actually OD stands for Over drive. Means in Over drive mode (in the highest gear could be 4th or 5th or any number) wheels spins faster than the engine (in technical terms Wheel RPM > engine RPM). Normally old days with 4 speed gear boxes when the car is in top gear(4th) engine revolving RPM and wheel revolving RPM used to be the same. Correct me if I am wrong

Cheers

quite true but wanna make a slight correction as it'll lead to misunderstanding..

OD means not Wheels RPM > Engine RPM .. it should be Final Drive RPM > Engine RPM ... and final drive and wheel spinning has a ratio, therefore it further drops down the RPM of wheels depending on the final drive ratio.

Edited August 21, 2007 by harshansenadhir

[ash]

Overdrive (OD) is the highest gear in an A/T vehicle. Most of the AT cars have only 3 speeds and a 4th (Overdrive).

More impoertantly run the OD on mode, that will save you a lot of juice. But in certain roads (uphills ro downhills) you might need switch off the OD. And if you’re in a hurry to pass other vehicles or want more response, switch off the OD. But never drive long distance with OD off.

As soon as we bought the FB15, I drove to Katunayake with OD off and when I returned the tank was half empty, and on the drive I could feel the high revs even at 100kmph. Not smooth at all. Then only realized what the OD button is for.

[Parak]

  harshansenadhir said:

quite true but wanna make a slight correction as it'll lead to misunderstanding..

OD means not Wheels RPM > Engine RPM .. it should be Final Drive RPM > Engine RPM ... and final drive and wheel spinning has a ratio, therefore it further drops down the RPM of wheels depending on the final drive ratio.

Thanks for correcting me I thought it otherway around. Anyway what is final drive?

[nira74]

  Parak said:

Thanks for correcting me I thought it otherway around. Anyway what is final drive?

Final drive is the RPM coming out of the Gearbox. (before the Diff)

[Mean Green z28]

let me add to the story ... in addition to the overdrive (where the output shaft of the transmission is rotating faster than the input shaft from the engine) on newer automatic transmissions, the torque convertor has a lockup feature at cruising speeds that makes it basically like a clutch. Theoretically, if the clutch isn't engaged and the auto trans is in its highest gear, the engine is turning faster than the transmission input shaft since it's only a fluid couple in the torque convertor ... when the torque convertor locks up, it makes it like a regular clutch and therefore the engine and the tranny input shaft turn at the same speed ... although this isn't a complete gear, it "simulates" another gear. When you're moving and you lightly get on the pedal, you'll feel it disengage. Press harder and you'll finally feel it shift into a lower gear. When towing a heavy trailor or climbing a hill, you should usually turn it off since the transmission will constantly try to get it into overdrive and engage the clutch (to save fuel) but the load will make it shift back and uncouple ... over and over and you'll burn up your convertor/clutch pack. But don't turn it off for too long 'cos you'll burn up your tranny by heating the fluid up too high.

Edited August 22, 2007 by Mean Green z28

[miniace]

Let me tell you why the OD was introduced and will try to minimize the technical terms here.

Mainly OD switch is not meant for SL.In Auto transmission (AT) the down shift from OD gear to next gives a a unpleasent jerk to passengers ,we call, "shift shock".This is hated by many.Especially during short drives.To avoid that,the answer came in as OD optional button.

For SL, AT cars with OD optional switch,leave it ON all the time.

MINIACE

[Silver]

In a nutshell...

The OverDrive (OD) button enables the car to shift into the highest gear (this won't happen if the button is disengaged). This is useful when you are travelling at a moderately-high steady-speed. With the OD enagaged the engine will rev at a lower speed while the vehicle is travelling at the same speed. Therefore, it lowers the fuel consumption.

i.e. Say that you're travelling at a steady 70 km/h. If you have the OD disengaged the engine will run at say 2200 rpm. But if you switch on OD, the automatic transmission will switch to the highest gear. Then the engine will run at less than 2200 rpm.

As others have said, if you're driving uphill/downhill, you better keep the OD disengaged.