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Old 10-21-2009, 05:55 PM   #1
originalJD
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Whats my gear ratio in this gm 12 bolt???

I bought this GM 12 bolt from a guy for 20 bucks. Couldnt pass it up cause the price. So I got it home, cracked open the case and counted the teeth. RING 43 AND PINION 13. My problem here is I can't find this count on the chart I use. So I thought maybe one of you guys could help me. Thanks. The chart I have is all I have. See below.



http://www.drivetrain.com/parts_cata...d_pinions.html

http://www.drivetrain.com/parts_cata...d_pinions.html
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Old 10-21-2009, 06:08 PM   #2
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Re: Whats my gear ratio in this gm 12 bolt???

normally you just divide the ring gear teeth by the pinion teeth which in your case comes out to 3.30 but i have never ever hears of that ratio in a 12 bolt. a recount might be in order. also there should be a stamp on the edge of the ring gear that will say the tooth count. the most common gear would be 3.07 which comes to 40-13
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Last edited by cableguy0; 10-21-2009 at 06:09 PM.
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Old 10-21-2009, 06:11 PM   #3
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Re: Whats my gear ratio in this gm 12 bolt???

Thanks. I'm pretty sure I have the right count. But I'll check again. I looked for stamps but didnt have a whole lot of time. It started to rain.
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Old 10-21-2009, 06:37 PM   #4
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Re: Whats my gear ratio in this gm 12 bolt???

Is this a 12 bolt car rear? if it is it could have that gear from the factory i was thinking truck rears
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Old 10-21-2009, 07:34 PM   #5
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Re: Whats my gear ratio in this gm 12 bolt???

Yes. I do beleive it is a car rear. It is has five lug axles too. Kinda the main reason I bought it. I have a 12 bolt in my 66 pu. But it is a 4.11 (I think) and 6 lug. So I may use the gears and axles in my truck
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Old 10-21-2009, 07:37 PM   #6
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Re: Whats my gear ratio in this gm 12 bolt???

The problem your going to have is car and truck lug patterns dont match. so if you upgrade to front disc brakes and 5 lug the wheels would have to be different front to rear. car is 5 on 4 3/4 and truck is 5 on 5. car and truck parts arent interchangable. the ring gear size is different as well as the axles.
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Old 10-21-2009, 07:44 PM   #7
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Re: Whats my gear ratio in this gm 12 bolt???

I realize that the patterns are different. I have a set of front disk brakes for a camaro. And one day when I win the lotto or something, I can get me some drop spindles to hang them on. I'm just kind of gathering parts as I go. Who knows, I might get this thing done one day.
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Old 10-22-2009, 06:38 PM   #8
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Re: Whats my gear ratio in this gm 12 bolt???

i'm gfoing to go out on a limb and guess....
It is more likely that you mis-counted the pinion than the ring, so....
43 is correct along with a 14 tooth pinion, you get 3.07
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Old 10-22-2009, 10:00 PM   #9
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Re: Whats my gear ratio in this gm 12 bolt???

Does the cover look exactly like the one under your '66 or is it a little different? If it is in fact a passenger 12 bolt, they were available with 3.31 gears. Otherwise you may have mis-counted

Quote:
Originally Posted by originalJD View Post
Yes. I do beleive it is a car rear. It is has five lug axles too. Kinda the main reason I bought it. I have a 12 bolt in my 66 pu. But it is a 4.11 (I think) and 6 lug. So I may use the gears and axles in my truck
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Old 10-22-2009, 10:51 PM   #10
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Re: Whats my gear ratio in this gm 12 bolt???

I had a 3.31 in a 1968 Camaro.
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Old 10-23-2009, 05:49 AM   #11
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Re: Whats my gear ratio in this gm 12 bolt???

My 12 Bolt out of a '78 3/4 chevy truck had 3.42's.
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Old 10-23-2009, 09:32 PM   #12
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Re: Whats my gear ratio in this gm 12 bolt???

Instead of counting teeth, does it not have the number stamped into the ring gear. That is where I look first, Spin the ring gear and look for the numbers, If it is 43 ring 13 pinion, it will have a number like 43:13 stamped on it. That will end all the cornfusion. find those numbers divide them, and that will be what your ratio is.
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