OK, I know the reason for overdrive is to slow down motor rpm for highway driving but I don't understand why high gear isn't 1 to 1 with a corresponding change in the final drive ratio to obtain exactly the same overall drive ratio as an overdrive transmission.

Why speed it up in the trans (which is what our 5th gear does) and then slow it down with the final gearing? Must be a good reason since every company does it this way but I don't get it. Is it all just marketing so they can claim "overdrive"?

 

ummm..... you use overdrive if you are hauling a lot of weight and you need more low end torque when climbing or decending a mountain. Overdrive can also be used in bad weather when you are driving slow and you dont want your car to shift too high. Ive never heard of overdrive for slowing your car on the highway?

 

He's refering to drive line speeds. He's aware of what overdrive is for. He's just curious as to why the way Evo goes about it this particular way.

 

Overdrive is for fuel economy. What you were talking about is low gears in an auto tranny. I think the original poster wanted to know why they accomplished the overdrive in the way that they did.

 

Actually you disable overdrive to achieve those goals.

Overdrive is when your gearing ratio is less that 1:1 (typically .75:1)

Main reason it is done over all is to quiet your engine at highway speeds, slow your engine down from a wear standpoint and to increase fuel efficiency.

 

It has to be done in the transmission as that is the only place that the gears ratio's are changed. Unless I am reading this wrong are you asking why dont the have a diferential that changes gears?

Maybe I am not getting what you are asking.

 

Hope your not a transmission mecahnic. You have it a bit backwards. If you had a car/truck in over drive and were hauling too much of a load, it would be bye bye tranny.

Over drive is for fuel economy and engine noise reduction at highway speeds. The term overdrive means the engine is turning lower RPM than that of the final drive.

BTW..Dale you beat me to it.

 

That's right. Not just EVO's either. I had a Corvette where 5th and 6th were both OD. I'm just curious by nature and don't understand why they do it this way. I checked out howstuffworks.com but they don't cover overdrive.

 

Interesting question, since if they made 5th 1:1 and changed the final drive to compensate, the drive shaft would be spinning slower too, right?

Speaking of overdrive, my dad used to have an old Chevelle, like a 68 or something. It had a knob you could pull that was labeled "Overdrive". Always wondered what that thing did...

 

that knob probably was an overdrive lockout so that you wouldnt go into the final gear or it was an overdrive engage so that you could only go into overdrive if it was pulled out.

Yes is corvettes the last 2 gears are OD gears (.9:1 and a .75:1 I believe, or somewhere close). This is their attempt to salvage some highway mileage.

 

No one can seem to answer the original question. Why use the trans to overdrive rather than just leaving it 1:1 with a lower ratio diff?. Why run the heck out of your diff through the gears when you can do it in the trans? I would like to know the answer too as it seems wierd yet all 5 and 6 speed ods I have seen do it this way.

FWIW The old fashioned way to get OD was to use a planetary gear system directly behind the trans to speed up the drive line, a knob or switch engaged the system usually by hydraulics that pushed the cone clutch together that then drove the planet gears. I had one on my (even then archaic "62" Austin Healey 3000).