This is probably a stupid question but can someone explain about and auto tran with overdrive. I want to know such things as when it goes into OD, why it does, what's the difference between driving in drive and OD, does overdrive give you better gas milage. Also, what are the benefits of a tranny with overdrive compared to one without. Thanks for any info.

 

All overdrive means is that the input from the engine is turning slower than the output (.7 to 1). This way the engine RPM's can stay low and keep your car driving at fast speeds like 60+. Older automatics and 4 speed manuals are turning 1 to 1, this means that in top gear these trannies' output at the driveshaft is turning at the same speed as the input from the motor. Automatic means the tranny will shift into overdrive automatically then the computer in your car will lock a clutch disc in the torque coverter which stops the slipping and lowers the RPM even more.

 

This is a really good explanation. Most cars shift into overdrive at around 35mph under normal cruising rpms. The whole point is to maximise speed and minimize engine rotation whichs cuts back on wear and adds gas mileage. It is not a gear for accelerating in and thats why it kicks out of it when you hit the gas.

 

I've been wondering about this too, although with regards to acceleration more than anything else.

Is the overdrive basically just a 4th gear then?

For example, if I am sitting at 50mph in Drive and nail it the car kicks down and goes.
If I were to sit at 50mph in Overdrive and then nail it would ther car accelerate just as fast?

ie in both examples would the car kick down to say 2nd?

In which case I may as well drive everywhere in Overdrive surely? but I heard someone say that was bad for the tranny...

If Overdrive is just a 4th gear why don't they label the shift gate 1, 2, 3, 4 instead of 1, 2, D, OD

???

It gave me the false(?) impression that overdrive was a .7 multiplier that kicked in above 40mph regardfless of the gear you were in...

 

Yes you could look at it that way.

Yes the car would kick down. It is ni different if you are in a manual car and downshift from 5th or 6th down to 3rd or a lower gear. Doesn't matter if you are in your overdrive gear you can still downshift.

Its not bad for the tranny and you should drive with your car in OD. Thats what it is there for. It is your cruising gear to maximise traveling distance and fuel economy while minimizing engine rotation.

They label it like that because 3rd gear in an auto is 1:1. Menaing that for every crank rotation the output on the tranny spins the same speed. Your drivetrain is spinning as fast as the motor. OD is labeled as such because you are beyond or "over" that 1:1 ratio and now are turning the output on the tranny faster than the engine speed.

Does this help at all?

 

Yes it helps loads.

You've cleared up my confusion.

I was always shifting from OD into D before doing any spirited driving thinking I got better acceleration...

 

The only differnce would be the time it takes the tranny to shift. I am not sure if the auto cars shift as fast out of overdrive as they do for other gears. Even still, we are only talking about fractions of a second. Nothing to worry about on the street.

 

If you have a shift kit in the automatic, the line pressure in drive is more than OD and same for first and second. My tranny shifts like stock in OD but if I forget and put it in D it barks the tires everywhere when it shifts or just hooks and makes my head hit the seat. I think that in a stocker it doesn't matter, the shifts are always the same, someone correct me if I'm wrong(Shifty).

 

Yea you are right and the added line pressure shifts the tranny faster. The time difference I was refering to was not the 1, 2 to 3 shifts (that should be the same time wise) but the OD shift when the converter actually locks. I would imagine this takes a little longer than the other shifts. There is a different shifting process for getting into OD so I assume there is a bit more lag. Doesn't really matter because if you are ever racing full bore and need to go into OD to go faster than you are in more danger than losing the race. That is of course if you have the power to do so.

 

Overdrive is a fourth gear. There is a set of clutches and a band that must be actuated in the transmission for teh car to shift into OD.

When the car shifts into 4th it applies the 2-4 band and releases the overrun clutch. (this allows engine braking). The lock up converter is electrically activated by the TCC solenoid. This sloenoid opens a valve that activates a clutch within the converter that stops slippage.

Driving In OD is no problem as long as teh car is not "hunting" between 3rd and fourth. This usually happens on slight grades at certain low speeds. If teh car hunts I drop mine into 3rd just for my sanity.