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Old 04-30-2005, 04:26 AM
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What does Overdrive do ?

got a question here. some cars (mainly american) have this "overdrive" button usually on the gear knob. what exactly does it do ?
from what i umderstand, it cuts off the final gear (i.e in a 5 speed tranny, it will only permit you to shift to 4th) and it kinda holds shifts longer. is that correct ?

and generally speaking, in automatic gearboxes, is it possible for the gearbox to have variable gear ratios ? does D3 mean longer gear ratios than shifting to D4 ?
or am i mixing gear ratio with holding a gear ?
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Old 04-30-2005, 04:39 AM
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youve got it right on the overdrive, i know on my mums camry its a 4 speed, but on the shifter its only L, 2 and D, so to lock it in 3rd, you need to put it in D and press the overdrive button.
As for D3 and D4, thats just locking it in 3rd or 4th as well
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Old 04-30-2005, 04:42 AM
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An overdrive unit is a secondary gearbox fitted after the main one. Usually main up of an epicyclic geartrain and operated electrically. It gives a direct drive (1:1) ratio when turned off; when turned on, it gives a higher output speed than the input. So, when turned on it allows you to cruise with a lower engine speed. It basically gives you more gear ratios, although on some cars it will only allow the overdrive to be used on certain gears ie on top to give you the higher 'cruise' ratio. Since 5-speed boxes came in, with that extra top ratio the overdrive was redundant.

Not quite sure on your second question. If i underatnd you, the D1, D2, D3 etc setting hold the gearbox in a fixed ratio, usually to stop the gearbox inadvertantly changing up when climbing a hill.
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Old 04-30-2005, 04:46 AM
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eh ?

Over drive is normally and ADDITIONAL gearing after the main box which extends the top gear ratio by a further ratio. usually inteded for motorway cruising as it drops the engine revs for a given speed.

What you describe is more akin to a towing-mode for the auto.

Where is it described as "overdrive" - is this another example of confused terminology betweeen US and the rest of us ? Like bonnet and hood

See http://auto.howstuffworks.com/automa...nsmission8.htm
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Old 04-30-2005, 06:37 AM
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Yeah... what Matra said It's just for lower fuel consumption if you don't need as much power (lower revs).
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Old 04-30-2005, 07:34 AM
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Quote:
Originally Posted by Matra et Alpine
eh ?

Over drive is normally and ADDITIONAL gearing after the main box which extends the top gear ratio by a further ratio. usually inteded for motorway cruising as it drops the engine revs for a given speed.

What you describe is more akin to a towing-mode for the auto.

Where is it described as "overdrive" - is this another example of confused terminology betweeen US and the rest of us ? Like bonnet and hood
and BOOT and trunk
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Old 04-30-2005, 10:00 AM
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I think in most cars overdrive is something that lets you stay in a sertain gear longer before shifting up (automatic). This allows you ot get higher hp/torque numbers as well as letting you tow heavier loads outside of the 1-2 settings.
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Old 04-30-2005, 10:01 AM
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Quote:
Originally Posted by targa
and BOOT and trunk
HOOD and trunk.
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Old 04-30-2005, 10:05 AM
gregvl52 gregvl52 is offline
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Cars fitted with overdrive (usually large vans and such, at least that I've seen) usually engage it automatically: thus, the button labeled "overdrive" would be to turn off overdrive.
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Old 04-30-2005, 10:19 AM
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Quote:
Originally Posted by gregvl52
Cars fitted with overdrive (usually large vans and such, at least that I've seen) usually engage it automatically: thus, the button labeled "overdrive" would be to turn off overdrive.
not on any car I've seen overdrive

Normally overdrive is limited to the top gear and so most folks leave it on.

There is confusion with folks thinking the option to delay an auto switching up a gear is "overdrive" it is NOT
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Old 04-30-2005, 12:24 PM
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Quote:
Originally Posted by Matra et Alpine
not on any car I've seen overdrive

Normally overdrive is limited to the top gear and so most folks leave it on.

There is confusion with folks thinking the option to delay an auto switching up a gear is "overdrive" it is NOT
I don't quite get what you're saying. On these large, fairly modern vans, the automatic transmission would choose when to go into overdrive in the top gears only for highway cruising. What is wrong with that? People might want to disable it if, for example, they are towing something and want more power available. See this post:

Quote:
Originally Posted by Some other forum
I have a Windstar from about the same period. The button on the end of the shift lever does disable the overdrive. On the Windstar, there is a light on the handle of the shift lever that lights up when the overdrive is disabled. Your manual should reveal where the overdrive light on your Aerostar is.
I'm pretty sure this is a real overdrive, not just the highest gear. Admittedly, there are people who would call a top gear where the ratio is something like 0.9xx:1, such that the engine is "overdriving" the transmission, an overdrive, but I'm pretty sure that modern automatic vans have overdrive that engages automatically, decided by the transmission computer, and, like just about any electronic system, can be disabled.

Last edited by gregvl52; 04-30-2005 at 12:26 PM.
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Old 04-30-2005, 01:31 PM
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I've always heard the simplest definition:

An overdrive gear is just one with a drive ratio greater than 1:1, and nothing more.

I may be wrong though, apperently. Matra always unleashes an earful
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Old 04-30-2005, 05:42 PM
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Quote:
Originally Posted by Matra et Alpine
There is confusion with folks thinking the option to delay an auto switching up a gear is "overdrive" it is NOT
thats what i thought it did. with overdrive turned off, the tranny will hold gears longer.
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Old 04-30-2005, 07:05 PM
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A TRUE overdrive as has been explained is an ADDITIONAL gearing on the output shaft of the gearbox which further increases the drive shaft speed for the same engine speed.

IT is NOT a "gear" in the gearbox. Those are called "gears"

The optoin for an automatic to delay the point where it automatically switches up a gear is NOT OVERDRIVE - even if at some point some noob nerd thought it did and the word has passed around. They're wrong An automatic box can decide for a variety of reason to shift up or down at different rev ranges. These are often in the user manual as "heavy load conditions".

So recap. OVERDRIVE is an EXTRA gear system in series with the gearbox as BjD explained in http://www.ultimatecarpage.com/forum...53&postcount=3
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Old 04-30-2005, 08:01 PM
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Matra has it right, anything holding gears longer would usually just be called sport mode or something like that. That's certainly what Volvos do. But yea, with the 5 and 6 speed autos around now, and especially with CVTs, overdrive is unnecessary.
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