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Old 03-30-2021, 08:25 PM   #1
apexone4697
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Default 3.9 or 3.54

I have a 2009 wrx. i have a 05 usdm sti rear diff which is 3.9 f.d. i want to swap a 6spd. i am reading different things about the reduction ratio. If a 6spd has a 1.1 t.r. does that make the rear 3.54? if i put in a 1.1 6speed, is it hard to swap pinion and gear in rear diff? can it be done without having to remove diff from car?
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Old 03-30-2021, 11:19 PM   #2
snow_bound26
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Probably the easiest option is to sell the diff and just get the GR 6 speed. Going in and swapping stuff to make it work may end up costing more money than you think. Unless you have the proper tools and know how for working on a trans.
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Old 03-30-2021, 11:32 PM   #3
MortonCharbo
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Quote:
Originally Posted by apexone4697 View Post
I have a 2009 wrx. i have a 05 usdm sti rear diff which is 3.9 f.d. i want to swap a 6spd. i am reading different things about the reduction ratio. If a 6spd has a 1.1 t.r. does that make the rear 3.54? if i put in a 1.1 6speed, is it hard to swap pinion and gear in rear diff? can it be done without having to remove diff from car?
1.1:1 will need 3.54 (1.1x3.54=3.894)
1:1 will need 3.9

The swapping of parts is not hugely difficult, but you will need to set gear lash. If you've never done it before, it might be kinda tricky. I also wouldn't attempt it with the diff in the car.
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