What does Overdrive do ?

[Misho]

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 ?

[fpv_gtho]

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

[BjD]

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.

[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

See http://auto.howstuffworks.com/automa...nsmission8.htm

[targa]

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

[PsychoChimp22]

and BOOT and trunk

HOOD and trunk.

[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.

[Matra et Alpine]

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

[targa]

HOOD and trunk.

no, those arent the same, the hood is the front, the trunk is the back

he said hood and bonnet, which are the same, but from the UK and USA respectivle

so i said boot and trunk, the boot being what they call our trunk over here

[fa22_raptor]

Yeah... what Matra said It's just for lower fuel consumption if you don't need as much power (lower revs).

[PsychoChimp22]

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.

[Matra et Alpine]

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

[aNOBLEman]

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

That's what I thought. Basically it gives you better gas mileage at highway speeds. The thing that makes shift points different and quicker is on large SUVs and trucks for towing. It makes the shifts at different rpm's so that the engine stays in the optimal power and torque curve.

[SonyAD]

An overdrive device is what Matra said.

An overdrive gear is any gear above that within which the vehicle is able to attain maximum designed speed (excluding electronic governors or speed limiters).

An overdrive gear is NOT a gear with a demultiplication ratio of less than 1:1, like a bazillion people dead wrongly believe and will happily 'educate' you.

THE VAST MAJORITY OF CARS DO NOT HAVE AN OVERDRIVE GEAR OR OVERDRIVE DEVICE.

It is amazing what little chance the truth usually has against misinformation and disinformation.

[pimento]

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.