I need your opinion???????????????????????????

[NOKTURNAL]

I am trying to explain to a fellow mechanic the basics of overdrive.

this is what he says "that going into overdrive shifts your transmission into a lower gear so you can accelerate faster, it switches, for example, from 4th to 3rd to get you from 65 to 75, and when you let off the gas, you are back in 4th at 75, if overdrive is off, or not there, your acceleration at higher speeds, or in cruising gears (4th, 5th) is slower because those gears do not have high acceleration rates" ----->> I totally disagree with that, so i need other mouths, to help explain the reason why overdrive is thier and what it does.

I tell him that overdrive is a final gear in the tranny that allows the car to cruise at highway speeds with lower engine RPM(which also saves MPG). and has nothing to do with the tranny kickdown

Am I wrong? i need your advise

How does Overdrive work.

thanks in advance for all replies

[mistamatrix]

His definitions is mixed up.

[brink]

Yah, it's a little screwed.

Think about it this way: as you go up the gears the transmission gets closer and closer to turning at the same speed the engine turns, e.g. for every revolution of the flywheel, in first the tranny turns 0.5 revolutions, in second 0.75, in third 0.99. Overdrive would be fourth gear where the tranny turns faster than the flywheel, like 1.25 revolutions for each 1 turn of the flywheel... it's overdriven one might say.


(note: those are not real gear ratios, just numbers for example purposes.)

[NOKTURNAL]

I cant wait to show this topic to him, anyone else have some say in this?


thanks all

[alltrac165]

Here's a link,
http://www.high-impact.net/transmiss...nderdrives.htm

[NOKTURNAL]

thanks all, for the help!!!!!!!!!!

[SpectraBlueCam]

If your friend thinks of a manual transmission car, the concept is not so hard to grasp.

On a typical 5-speed manual transmission car, gears 1 to 3 are underdrive gears, 4 has a 1 to 1 drive ratio, and 5th is overdrive.

Translate this to a typical 4-speed automatic transmission (yes, some newer ATs have 5 gears, but the concept is the same). 1st and 2nd gear are underdrive gears, 3rd or "D" is 1 to 1 drive ratio, and "OD" is the overdrive gear.

Having the OD on (with a button on Toyotas and Nissans, and putting the lever is "D4" in Hondas, etc.) means the automatic transmission will have all 4 gears at its disposal. Whether it kicks down or not is a matter of the ECU to decide, based on the gear you're in, your speed, and the amount of change in the throttle position. That said, if you are already in OD, it is likely that your car will kick down if you mash the throttle. If the OD is off, your ECU may decide that you are already in the optimal gear for the desired speed increase.

Thinking back to a manual transmission car, switching the OD off just is the equivalent of never shifting into 5th gear, regardless of speed.