Author Topic: Finding rear end ratio  (Read 4212 times)

Offline Jerr-conn

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Finding rear end ratio
« on: February 16, 2017, 02:47:23 PM »
I thought I knew how to find rear end ratio in my 87 Chevy s.b. 2 wheel drive. It does NOT have posi.I marked driveshaft and marked tire. I started to turn shaft and about 1/2 way around the tire
Stopped turning. What happened? My speedo reads faster than I'm going. How fast I don't know.
I thought if I knew the ratio, along with my 28" tires, I could use one of the calculators to
Determine what gears I would need. Guess I need some help. Oh, it's a 350 / 700r4 if that
Makes a diff.  Thanks.
Jerry

Offline FlatBlack77

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Re: Finding rear end ratio
« Reply #1 on: February 16, 2017, 03:37:21 PM »
I think you have to turn the tire backwards for open diff and forwards for posi trac.
"When you are a hammer, everything looks like a nail"
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Offline Jerr-conn

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Re: Finding rear end ratio
« Reply #2 on: February 16, 2017, 04:56:57 PM »
As noted, my truck has no posi. When up in air, one wheel turns forward and one turns backward.
What do I do now?
Jerry

Offline Irish_Alley

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Offline Dr_Snooz

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Re: Finding rear end ratio
« Reply #4 on: February 16, 2017, 09:09:03 PM »
Oo oo! It's really easy. You pop the cover off the diff and count the teeth on the diff. Then drop the drive line. Remove the pinion and count the teeth on it. Then look up the ratio in a chart online.

See? Easy!

LOL
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Offline hatzie

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Re: Finding rear end ratio
« Reply #5 on: February 17, 2017, 06:56:26 AM »
If the axle code stamps are readable you can decode the three digit alphanumeric axle build code by using the tables in the GM parts books. 
I scanned all six 1973-1991 books as searchable PDF files.  You can download them from the link in my signature and type the axle code into the Search box in Adobe Reader.
You can search the online versions of my scans and more scanned GM parts books using the search function on Ian Harding's site http://gmpartswiki.com/query
« Last Edit: February 17, 2017, 06:59:24 AM by hatzie »
SVC & wiring mans --> Here http://tinyurl.com/7387BRD-SVCMAN or My Bucket @ http://tinyurl.com/SQ-SVCMAN
Parts & Illustr Books -->http://tinyurl.com/SqParts
GMSTG Textbooks-->http://tinyurl.com/STG-TEXTBK
Radio Manuals-->http://tinyurl.com/DELCORADSVC

Offline blazer74

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Re: Finding rear end ratio
« Reply #6 on: February 17, 2017, 07:43:53 PM »
Open diff procedure to determine ratio. Jack one side only.

Turn tire full 2 revolutions while counting drive shaft revolutions.

As in my case 4:10 gears, 2 revolutions of tire my shaft turns approx 4 1/8 turns.

I've done it and it works.

Like was said also, code on passenger axle tube if the gears have not been changed will give ratio.

Offline Jerr-conn

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Re: Finding rear end ratio
« Reply #7 on: February 18, 2017, 07:09:33 AM »
O k . I understand procedure now. But, why do you only have to turn a limited slip rear end
Only 1 turn to get the ratio. Is it the spider gears or something else that makes this way of
checking Different ?  I always wondered about this. Explanation please.  Thanks
Jerry

Offline Jerr-conn

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Re: Finding rear end ratio
« Reply #8 on: March 02, 2017, 11:24:29 AM »
O K.   I used the open differential procedure from BLAZER 74. It worked great and came out to
3:08, same as RPO code on glovebox door. Thanks again BLAZER74.
Jerry

Offline ehjorten

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Re: Finding rear end ratio
« Reply #9 on: March 02, 2017, 05:04:41 PM »
In a posi you have to overcome the friction in the discs.  When you jack up the rear and spin it, it is like a locked spool so you get the proper number of revolutions.  In an open diff the spider gears just spin and the other tire goes backwards.  You have to stop the other tire from spinning like other have said.

I have found the easiest and most reliable way is to pull the cover and rotate the ring gear until you can read the stamped ratio on the side of the ring gear!!!  Plus...when was the last time you changed oil in the diff?
-Erik-
1991 V3500 - Gen V TBI 454, 4L80E, NP205, 14 bolt FF, D60, 8" Lift on 35s
1977 K20 Silverado - 350, THM350, NP203, 14 bolt FF, D44, Stock Lift on 31s
1969 Chevelle Malibu Sport Coupe - EFI350, THM350
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Offline Irish_Alley

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Re: Finding rear end ratio
« Reply #10 on: March 02, 2017, 05:40:14 PM »
In a posi you have to overcome the friction in the discs.  When you jack up the rear and spin it, it is like a locked spool so you get the proper number of revolutions.  In an open diff the spider gears just spin and the other tire goes backwards.  You have to stop the other tire from spinning like other have said.

I have found the easiest and most reliable way is to pull the cover and rotate the ring gear until you can read the stamped ratio on the side of the ring gear!!!  Plus...when was the last time you changed oil in the diff?
If you canít tell yourself the truth, who can you tell it to?~Irish_Alley

When you have eliminated the impossible, whatever remains, however improbable, must be the truth ~Sherlock Holmes

Offline Jerr-conn

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Re: Finding rear end ratio
« Reply #11 on: March 03, 2017, 08:50:15 AM »
I've only had the truck a couple months and it's been cold, so I  only have solved a few electrical
Issues, changed cooling system hoses, replaced all engine belts. Replaced rear bumper with new
Sport type purchased from this site. I'm going to change rear end lube and grease front wheel
Bearings. I have a nice heated workshop in my pole barn, but the back is not heated, so l have
To pick decent days to work on the truck, but it's coming along fine. I also plan on lowering it also.
Maybe 2/4.
Jerry