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  #1  
Old 01-11-2013, 06:46 AM
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Final drive questions

Let's throw out some typical final drives: 3.545, 3.7, 3.9, 4.11, 4.44 ...

1) Is this an exchange in the final gearset, most often the differential?

2) Does this ratio mean that i.e 3.545 rotations of the flywheel means one rotation of the driven wheel?

3) Is final drive always engaged or is it "overdrive" after the 4th gear. i.e will the 3.545:1 ratio only stand true when the transmission (4EAT here of course) is in 4th gear?

If such is the case, then 3.545:1 means lower RPM at high speeds = less noise, less fuel consumed compared to 4.44:1 ?

I read here somewhere that 4.44 gave better accelleration. Does this mean that the higher final drive ratio, the shorter the gears are?

Why does 3.90 seem to be a magic number? Because it's a tradeoff between comfort and performance?
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Old 01-11-2013, 07:31 AM
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Re: Final drive questions

1. Final drive is the ratio of the ring and pinion gears in the differential and is technically independant of the transmission gears themselves. I say technically because in Subarus the front diff is part of the transmission and therefore normally considered to be intermeshed with what ever vehicle the trans came from. In theory you could install the diffs from a different Subaru and changed your final drive ratio.

2. The ratio reflects the input/output of the differential. In the case of our 3.54 gears it means 3.54 rotations of the driveshaft for every rotation of the wheels.

3. There is no engaging or disengaging of "final drive", it simply exists as a function of the differentials.

The higher the number to more torque is effectively transmitted to the wheels (like being in a lower gear). The downside is that you willl be turning higher RPM at all times while moving, leading to higher fuel consumption.
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Old 01-11-2013, 12:22 PM
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Re: Final drive questions

Thanks. I'll look more on the web and see if I can calculate the flywheel-to-drive wheel relation
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Old 01-11-2013, 07:06 PM
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Re: Final drive questions

There are two ratios that determine the engine to drive wheel ratio. First is the transmission, ours will have four ratios. IIRC, the first two are underdrive, third gear is 1:1 and fourth is overdriven.
This will determine the speed of the propeller shaft to the rear and the shaft inside the transmission that drives the front diff.

The final drive is the ratio of those two shafts to the axles.

Somewhere on this site is a list of the 1-4 gear ratios for the US transmissions.

Just to clarify;
Engine RPM - 2000
4th gear ratio 1 : 1.15 (just a guess)
Prop shaft RPM - 2300
FD ratio - 3.545 : 1
Axle/wheel RPM - 649
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Old 01-12-2013, 12:49 PM
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Re: Final drive questions

Thanks!

I've seen most STI setups have 3.9 as final drive. Any particular reason for this? Some sort of sweetspot, or?
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