Help identifying rear diff - JeepForum.com
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post #1 of 7 Old 07-29-2021, 09:46 AM Thread Starter
Tundra-Honky
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Help identifying rear diff

I’m trying to identify the gear ratio of my rear differential. I lifted the jeep, put it in neutral and spun a tire. The other tire didn’t spin at all. As I spun the tire one revolution the rear drive line spun 2 1/8 times.

Any idea on rear end posi or open? Or other? And any idea on gear ratio?

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post #2 of 7 Old 07-29-2021, 09:46 AM Thread Starter
Tundra-Honky
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I should mention it is a modified 1980 with amc 20 rear end
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post #3 of 7 Old 07-29-2021, 09:09 PM
80cj
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80's came with 3.08 standard and 3.54 optional. Best thing to do is to pull the diff cover and look on the ring gear edge for the ratio or count the gear teeth. The vehicle is 40 years old and previous owner(s) may have changed the gears.
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post #4 of 7 Old 07-30-2021, 12:12 AM
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It is an open diff or a worn out posi. There are stamps on the axle housing to the left of the diff, otherwise you need to take teh cover off to have a look.

Could be 2.73:1 Stamped AA or D. If it had posi it would be DD.

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post #5 of 7 Old 07-30-2021, 06:39 AM
Axhammer
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Garage
You do not have to remove the rear end cover to determine the gear ratio of the gear-set that is installed inside.

You will have to remove the cover to verify what style differential is installed.

You probably have an open differential. A worn Limited Slip differential will often still spin the other side when in the air like you did it. It would have to be extremely worn out to not spin at all, in the air, which is a possibility.

Any markings, tags or stampings that indicate gear ratio, are only accurate if the rear is unmodified, or stock. Since your second post indicates it’s modified, that would further suggest the markings, tags or stampings are no longer accurate.

If you do the math using the numbers you provided, your rear end has 4.25 gear ratio. You said you spun the rear tire once on one side while the other side did not spin at all, and the drive shaft spun two and one eighth times.

2.125 spins of the driveshaft for 1 spin of one wheel, while the other side wheel does not spin.
2.125 x 10 = 21.25
21.25 x 2 = 42.5
42.5/10 = 4.25/1 gear ratio

Watch the link below and do it again. Your other side wheel that you said did not spin, might have spun slightly, and you didn't notice it, skewing the numbers. If you chock one side or only Jack one side, it will eliminate that possibility.


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post #6 of 7 Old 07-30-2021, 11:04 AM
80cj
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Looking at your profile, it looks like there's been some work done on the Jeep. You show that it has a T-18 trans which was never offered in an 80 model. The front diff has been changed to a Gleason (Torsen?)unit which is a gear driven LS. So it's very possible that you ratio is 4.10 as per Axhammer's calculation. I would still pull the cover off to see what's actually in there and change the lube.
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post #7 of 7 Old 07-31-2021, 05:00 PM Thread Starter
Tundra-Honky
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I liked the e idea in the video of 10 turns. Gives larger average. I came up with 4.16 so I suspect 4.10 gearing. Thank you for the help.
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