Overdrive ? - Information required please

[netarchie]

what does this mean "Overdrive on 4th and 5th gear"

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[tsk1979]

Overdrive - Definition
A transmission in which the highest gear ratio is less than a one-to-one ratio. This means the drive shaft turns faster than the engine crankshaft. The overdrive feature saves fuel and, because the engine runs slower, engine wear and noise are reduced.

A ratio of 1:1 means that your driveshaft is turning at the same speed as the crankshaft.
Overdrive means that the driveshaft(which goes to the wheels) is turning at a speed higher than the crankshaft(driven by the engine).

Most cars have overdrive in 5th gear and many in 4th gear too.

[Revvhead]

well i guess that means the both 4th & 5th are geared lower than 1:1, for these models.

Overdrive is basically any gear witha ratio lower than 1:1.

i.e at 1:1 ration for one rotation of the crank the wheels(driveshaft actually) should turn once,
in overdrive they turn 1.x times
Of course this is not taking into account differetial ratios etc.
before people come eatin my head.
Hope that sorts it out

Of Course

[v1p3r]

Erm, isn't overdrive when the output from the gearbox is spinning faster than the crankshaft? It can't be the final driveshaft, because those have a built in reduction ratio from the differential.

In most cars, 4th gear is normally 1:1 or very close.

[Revvhead]

Originally Posted by Revvhead

Of course this is not taking into account differetial ratios etc.
before people come eatin my head.
Hope that sorts it out

there there, i did say driveshaft actually.
and i meant like the driveshaft leading to the diff as would be found in a RWD.
was just givin the simplistic explanation

[netarchie]

So , it means in short the term had nothing to do with Speed of the car and rather it is for saving fuel.

[DaPilot]

Overdrive should be used on the freeway/highway. Not in city driving. If you use OD in city driving, you will cause more wear to your transmission and also not gain any fuel efficiency.

Over drive essentially adds an extra gear to your drive ratio effectively lengthening your final drive ratio and causing your engine to maintain lower RPMs. In other words... an overdrive gear lets the engine run at a lower rpm to maintain the vehicle’s cruising speed. This is perfect for cruising on open roads but bad (over time) for stop and go or heavy traffic conditions (definately a NO NO for Bombay City traffic). So if your car has OD its best to switch it off while driving in the city

[Deeps]

Well, its hard to switch off overdrive on an M/T unlike the A/Ts that you are probably used to in the US.

[DaPilot]

Originally Posted by Deeps

Well, its hard to switch off overdrive on an M/T unlike the A/Ts that you are probably used to in the US.

You are right it is very easy in A/Ts but at the same time if you think about it.. it is quite simple for M/Ts also.... just dont use the gear that has OD when you are driving at a low speed (you will hear a knocking sound)

As pointed out earlier... any gear that has a ratio lower than 1:1 is called an overdrive gear. It's usually the fifth gear on standard transmissions, and fourth gear on automatics.

[SLK]

Originally Posted by netarchie

So , it means in short the term had nothing to do with Speed of the car and rather it is for saving fuel.

Of course its everything to do with the speed of the car!
More speed less RPM thats why fuel saving.