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Old 10-09-2010, 04:02 PM   #1
miller6t9
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rear end, SPID, gear ratio question

ok...the SPID shows that my truck came with a G80 Posi-traction rear end (no other rear end options listed), the M49 turbo hydra-matic and the LS9 350 engine. Am I correct to assume the rear end ratio was a 3:73?
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Old 10-09-2010, 05:19 PM   #2
Longhorn Man
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Re: rear end, SPID, gear ratio question

nope, the 3.07 gearset was standard with the 350/automatic
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Old 10-09-2010, 05:27 PM   #3
chevytruckluver
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Re: rear end, SPID, gear ratio question

Quote:
Originally Posted by miller6t9 View Post
Am I correct to assume the rear end ratio was a 3:73?
Never assume anything, you should know better!
Longhorn is right, unless they got changed somewhere along the line
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Old 10-10-2010, 11:16 PM   #4
aintmisbehavinn
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Re: rear end, SPID, gear ratio question

If mine is a 307, why does it tach 2200 at 53mph? 350 automatic OEM
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Old 10-11-2010, 12:09 AM   #5
Longhorn Man
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Re: rear end, SPID, gear ratio question

i think a 307 was the exception to the rule, i think it may have gotten a 3.73.
However, 'taching out' (to me) indicates redline... 5500 or so RPM... ?
you may have a problem there.
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Old 10-11-2010, 01:12 AM   #6
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Re: rear end, SPID, gear ratio question

Best thing to do is pull the rear diff cover off to see what you really have. If you haven't ever changed the fluid it is a great time to do so. It is real easy to tell what you got once you are in there. you just wipe off the edge of the ring gear that faces to the rear of the truck and look for some #'s. If it is a 3.73 for example it will say "11-41", this indicates the ring has 41 teeth and the pinion has 11 teeth. Then you just divide the big # by the little #, so it would be 41/11=3.72727... so 3.73. It really doesn't take long to do, I checked 3 rears in the junk yard the other day and it only took 30 minutes (I didn't have to put them back together though ). Good luck!
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Old 10-11-2010, 08:30 AM   #7
miller6t9
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Re: rear end, SPID, gear ratio question

I was looking at the specification list on the '72 brochure (http://www.lov2xlr8.no/brochures/che...v/bilder/9.jpg) and best I could tell the 3:73 was the standard gear and the 3:07 was optional.
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Old 10-11-2010, 01:33 PM   #8
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Re: rear end, SPID, gear ratio question

Quote:
Originally Posted by miller6t9 View Post
I was looking at the specification list on the '72 brochure (http://www.lov2xlr8.no/brochures/che...v/bilder/9.jpg) and best I could tell the 3:73 was the standard gear and the 3:07 was optional.
There is something strange about that literature. Those are not the GM names for those colors.

Last edited by OhOneWS6; 10-11-2010 at 01:35 PM.
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Old 10-11-2010, 01:40 PM   #9
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Re: rear end, SPID, gear ratio question

if in doubt do your rear a favor and drain the 40yearold sludge out of it clean and fill with new. in the process you can take any guesswork out of the equation and ID your gears and carrier
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Old 10-11-2010, 05:15 PM   #10
Longhorn Man
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Re: rear end, SPID, gear ratio question

Quote:
Originally Posted by miller6t9 View Post
I was looking at the specification list on the '72 brochure (http://www.lov2xlr8.no/brochures/che...v/bilder/9.jpg) and best I could tell the 3:73 was the standard gear and the 3:07 was optional.
in a base model truck, that is true.
Once you get an optional engine, or tranny, or both, it may include a gearset in the rear as standard with that combination.
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Old 10-11-2010, 05:30 PM   #11
68C10HotRod
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Re: rear end, SPID, gear ratio question

Here's a copy of some 71 data that might shed some light.



Of course, I've already found a number of times the "parts" don't always match the SPID. As others have said, checking is always the sure way of knowing.
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Old 10-11-2010, 07:53 PM   #12
aintmisbehavinn
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Re: rear end, SPID, gear ratio question

It must be a 3.73 as the original owner quoted, it's not listed anywhere in the codes on the glove box door. I did change the fluid, I'll know for sure when it goes in for a limited slip diff.
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