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Hi Yenko1,
You either have the 7 1/4" rear (skeleton shaped cover) or the 8 1/4" rear (oval cover with rubber fill cap). I'm not sure but think the 7 1/4 only came with 4 cylinders. All the 3.9/5.2/5.9 should have the 8 1/4. One of the bolts on the cover should have a metal tag with a number on it. That's the ratio - 3.21, 3.55, 3.91, etc....
 

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On my 94, there is a sticker with the RPO codes on the back of the hood. That doesn't mean it still has it, but that's how it came from the factory. I don't remember if it gave the ring gear size, but I know it had the ratio.
 

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Tags on rear end.

I looked at my diff. and it has two metal tags. One says 3012 and the other has 2.9. It is a fleet GTE/Verizon 1988 reg. cab longbed 3.9 auto. I guess they were ordered like this for better fuel economy>????
 

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well, theres a sig. diffrence between 3.55 on up and 2.9, to make sure jack up the rearend, tranny in neutral, and rotate the drive shaft while counting how many times the tire rotates, it'll be hard to get an exact ratio that way but you sould be able to tell the diffrence between 3.55 and 2.9
 

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With posi this will work fine, with open rear the opposite tire will want to counter-rotate. Makes it hard to get a good count. If non posi, try leaving one rear tire on the ground. The other will rotate easily (posi will not turn with one tire down). The ratio will be half of actual with this method so multiply by 2 for the ratio. 1 1/2 turn driveshaft for one tun of the tire x 2 = 3.0. Good luck!
 
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