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An excerpt from a post by Cactus from the thread "Car"-kabob - https://idforums.net/index.php?showtopic=41824

The bolded part really made me puzzle. Am I suppose to put it in O/D before engaging in cruise control? Is that more fuel efficient? I searched around the net for an answer and it seems overdrive is for when you drive at highway speed (45mph+) and reduce engine wear. But the car manual doesn't mention anything about better fuel efficiency. Does anyone know about what overdrive on an automatic transmission really is? My car is a 2005 Toyota Corolla LE.


EDIT: Upon carefully reading the manual again, it does mention O/D is actually on at all time and that it achieves better fuel economy at higher speed! I've attached two images for illustration.





I guess I answered my own question. Regardless, if you know anything technical about overdrive on an automatic, share it here!

This post has been edited by Perry on Sep 18 2010, 12:43 PM

doesn't overdrive essentially "lengthen the gear ratio"? i guess you would be at a lower RPM at a higher speed, thus saving you gas?

thats my guess. i could be spot on or i couldve missed the dart board.

Overdrive changes shift points only. Causes the vehicle to work in a more fuel efficient manner. In your top gear, it wont do anything unless you're doing any heavy throttle usage, in which case a "non-od" car will drop down a gear where as an "od" car will do nothing but chug along.

In several cars that I've had, it dropped the revs at highway speeds.

And yeah, it's on by default.

I don't mess with automatic transmissions much, they're about as complicated as the space station.

Overdrive refers to any gear with a drive ratio greater than 1:1. In most transmissions from the days of yore most gear ratios were more than one to one, meaning the engine always spun faster than the driveshaft. As engines (predominately in V8 heaven America) became bigger and more powerful, they could gear things with more lazy ratios. Some early automatics, which had planetary gearsets could do 1:1 ratios. Direct drive is pretty nice, but somebody came along and added lower ratios and thus the overdrive was born. When you could drive on the highway at under 2000 rpm, you bet your ass that was a selling point.

My cars both have a 1:1 ratio for fourth and a <1:1 ratio for fifth.

Theres nothing complicated about automatic transmissions

Basic components a flex plate, a torque converter, a valve body (usually hydraulic, powered by a pump off the input of the transmission), and planetary gears. The flex plate and torque convert bolt together, and work like a fan blowing another fan. This is what spins the input shaft of the transmission In most automatics the "gears" are self contained "packs" and essentially "stack" inside the transmissions. Each gear is basically spinning at all times, but they dont engage until the valve body engages a particular gears clutch band inside the pack.

Rough basics, but should make it easy enough to understand.

No, I get that completely. It's the electronic controls most transmissions have nowadays that just baffle me. Also, why would you have about 34million hydraulic parts when you could just have a few dogs and selector forks?

Autos are stronger (depending on the length of the shift time and amount of planetary gears of course), (can) shift faster, shift smoother, and are much easier to operate for the average joe.

This post has been edited by sideways on Sep 21 2010, 05:31 PM

They also shift pretty much when you don't want to, some "manumatic" shifters take AGES to actually shift when you slap them, and they overheat and die much more often than a manual box.

Simplicity makes everything better.

I have paddle shifters on my car. About the only thing i really use it for is to put the car into top gear on the highway and leave it there. its response is too slow for any other use....

Man what cars are you driving

Mostly domestic and some azn stuff from 2006 at the newest back to about 1980. Also a couple 80's BMW's. Nothing I've ever driven has come close to handling as well as my 6er. Not even an E30. Also some trucks. Best automatic I'd have to say was in a 2007 I think Chevy 1500. There's something awesome about switching from 2wd high to 4wd low at 25 mph.

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Missed this one before...

They are also better at failing faster.

Certainly can if you dont take care of them properly. Sadly most people -never- replace the transmission fluid like they should. So many drivers dont think to touch the fluid UNTIL they start having transmission issues- and sadly by then often replacing the fluid does more harm then good since its the clutch material in that fluid thats making the transmission work.

How often are you generally supposed to change the transmission fluid? Brakes, steering, air filter, etc.?

This post has been edited by Kiroshino on Nov 9 2010, 03:59 PM

Do what the factory recommends. Manual Tranny oil lasts a lot longer than ATF. Brake fluid should be changed about every year or so, it's hygroscopic and boils much more quickly when you get even a little water in the system. Steering fluid usually doesn't have a change interval, just keep the level where it needs to be. Air filter fluid typically lasts 10-15,000 miles, less in dusty environments. Etc. fluid usually lasts about a day or so, you can drain it and refill it, or not.

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Same goes for sticks too.

However, sticks in general tend to outlast the car, can't say the same for automatics.

Cept youre not relying on toasted clutch material in the fluid to make it work that once you flush out just makes it worse Got some sexy synchro material, but thats fine to remove.

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Id have to say much more than likely that was the case.