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Old 12-06-2013, 10:16 PM   #41
David McMahon
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Join Date: Feb 2013
Location: Suffolk, UK
Posts: 1,451
My Ride: '99 E38 740iL
Quote:
Originally Posted by Rotten Robbie View Post
A dumb question, but I don't know how to tell what ratio my rear differential is...
I have a 325i 5speed, and at 70mph on flat ground I'm turning exactly 3000 rpm.
Is the differential ratio stamped somewhere, or encoded in the vin # ?

I think I need to replace it because it is making a clunking noise and seems to have a lot of play.
I especially notice it at low speeds on uneven ground.
I heard that something can break inside the diff, and cause this to happen. (?)
Try this http://www.bokchoys.com/differential...calculator.htm
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Old 12-08-2013, 12:59 PM   #42
Rotten Robbie
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My Ride: 01 325i
Thank you for the link! lots of good info. the chart said I have a 3.15, but the calculator gave me 3.38 (?)
Is the ratio stamped on the diff somewhere? I guess I could jack it up and measure the rotation of the axle per one rotation of the drive line.
However, there are quite a few ratios and it might be hard to tell exactly.
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Old 12-08-2013, 01:41 PM   #43
David McMahon
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Join Date: Feb 2013
Location: Suffolk, UK
Posts: 1,451
My Ride: '99 E38 740iL
Quote:
Originally Posted by Rotten Robbie View Post
Thank you for the link! lots of good info. the chart said I have a 3.15, but the calculator gave me 3.38 (?)
Is the ratio stamped on the diff somewhere? I guess I could jack it up and measure the rotation of the axle per one rotation of the drive line.
However, there are quite a few ratios and it might be hard to tell exactly.
I just clicked 325i and it gave a 3.15 and a 2968RPM @ 70MPH, also depends on your Wheel size. A 3.38 diff should return a 3185RPM @70 MPH
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Old 12-10-2013, 07:44 PM   #44
Rotten Robbie
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Join Date: Feb 2013
Location: Oregon
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My Ride: 01 325i
Yes, it probably is a 3.15, but my tire/wheel size was not listed either. Probably because my car is old, but I do have take offs from an 05 or 06 325, that are 16".
The gear ratio must be stamped on it, or somewhere on the car? Like how would I go to a junk yard and know what ratio a wreck has?

It's been snowing here in OR, so I'm driving my truck, but I'll write down the vin tonight and check out the web site.

Thanks for the link!
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Old 12-10-2013, 07:49 PM   #45
David McMahon
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Join Date: Feb 2013
Location: Suffolk, UK
Posts: 1,451
My Ride: '99 E38 740iL
Quote:
Originally Posted by Rotten Robbie View Post
Yes, it probably is a 3.15, but my tire/wheel size was not listed either. Probably because my car is old, but I do have take offs from an 05 or 06 325, that are 16".
The gear ratio must be stamped on it, or somewhere on the car? Like how would I go to a junk yard and know what ratio a wreck has?

It's been snowing here in OR, so I'm driving my truck, but I'll write down the vin tonight and check out the web site.

Thanks for the link!
Vin really helps, RealOEM has the part numbers for the different ratio diffs so they might be stamped on the Diff casing itself.

http://www.realoem.com/bmw/showparts...85&hg=33&fg=05

http://www.realoem.com/bmw/showparts...85&hg=33&fg=05
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