Author Topic: 10 bolt Posi gear ratio ?????  (Read 1196 times)

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Offline Tazz

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10 bolt Posi gear ratio ?????
« on: September 30, 2012, 04:29:24 PM »
I have a 10 bolt posi disc brake rear end in my 80 trans am parts car trying to find out the gear ratio is there numbers
 on the rear end sume were that will tell me there is a tag but it just ses the lube to use.

Offline gilbertsta

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Re: 10 bolt Posi gear ratio ?????
« Reply #1 on: September 30, 2012, 04:37:30 PM »
Mark the tire and the drive shaft..
Spin the tire one full turn and count how many times the drive shaft goes around.....i think thats how you do it

78 trans am /400/brown


Offline pancho400cid

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Re: 10 bolt Posi gear ratio ?????
« Reply #2 on: September 30, 2012, 08:31:31 PM »
The rear end housing should be stamped from the factory with a code...  Here is some info:

 http://www.78ta.com/rearcodes.php

If the gears are not factory, or you're not sure then ,as mentioned, you can jack up the car with the trans in neutral then turn the driveshaft and count how many turns of the DS it takes to make a full turn of the tires (both tire will rotate the same direction if it is posi).  The down side is it is hard to tell the difference between say 3.08 and 3.23.

As usual, if you're not sure, the most difficult option is the best... pull the cover and count the number of teeth on the pinion gear and the ring gear.  The ratio is # RG teeth / # pinion teeth
« Last Edit: September 30, 2012, 08:33:16 PM by pancho400cid »
1978 Trans Am - Brown - Current Project
1978 Trans Am - Silver - Future Uncertain

Offline Grand73Am

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Re: 10 bolt Posi gear ratio ?????
« Reply #3 on: September 30, 2012, 10:40:24 PM »
Yes, car in neutral and blocked, both rear wheels lifted, put a mark on one tire and a mark on the driveshaft. Turn the marked tire forward one rotation, and count the driveshaft rotations. A little over 3 times is 3.08, about 3-1/4 turns of the driveshaft would be 3.23, about 2-3/4 turns is a 2.73, and so on. My 1980 Pontiac service manual shows the ratios offered that year are 2.41, 2.56, 3.08, 3.23, and 3.42.

Here's a pic of where the code is on the housing. That's the front passenger side axle tube you see there.  It's stamped into the metal of the tube. You may be able to see it if you clean it off with a wire brush and little sandpaper. A flashlight and a magnifying glass might help read it. It'll be a pair of letters that's the code.

Steve F.

Offline Tazz

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Re: 10 bolt Posi gear ratio ?????
« Reply #4 on: September 30, 2012, 11:47:40 PM »
o ok cool thanks for the info will look tomorrow