Determine differential gear ratio - The 1947 - Present Chevrolet & GMC Truck Message Board Network

John Pasinski · 04-29-2012, 11:51 AM

I know this has been asked before, but can someone tell me how to identify the gear ratio on a posi (or semi posi) rear end? I"m asuming it's posi traction as there's a "raised" portion on the left side of the cover for a gear. I have not removed the cover as I had no tools at the time. It's in an '82 C20 deluxe with leafs. I'm trying to find a 3:73, 3:42 etc and swap out my 4:11. Please pardon my ignorance on this subject.

**LockDoc** · 04-29-2012, 12:13 PM

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There are several different ways. I'm not sure about the later ones but the '67-'72 have a code stamped on the pass side axle tube. You can also mark the tire and drive shaft with chalk and spin the tire and see how many revolutions the drive shaft makes. The best way is to remove the cover and check the numbers on the ring gear. That way you can check the posi situation and change the gear lube. I don't think the "raised portion" means anything.

The later rear ends will not fit in our trucks without modifications.

LockDoc

John Pasinski · 04-29-2012, 01:03 PM

Thank you sir, I knew there are a couple of old timers on the board that could help me. Are you saying an '82 won't swap into my '72 w/o changes?
Also, the way the truck is sitting, one wheel won't spin. So I am unable to do the "rotate the wheel & count" trick. Do all ring gears have numbers stamped into them?

**too much stuff** · 04-29-2012, 01:10 PM

If one wheel doesn't turn, it isn't posi. I don't think it is a direct swap either. Do you have coils? or leafs? How about the doner truck? And the "raised portion" won't tell you anything as they all had that.

**special-K** · 04-29-2012, 02:27 PM

Does the truck have a SPID? Optional ratios are listed. Is this rear in an '82 C/20? It could have a full floating or a semi-floating 14-bolt 8-lug rear. Is your '72 a C/20? The shock mounts and spring pads will need relocated. Most likely,if it's and automatic it will have 3.73s. If I were you I would get some tools. At least a 1/2" and a 9/16" wrench. You need tools to own an old vehicle.

John Pasinski · 04-29-2012, 03:59 PM

Special K......Pardon my grammar. Yes I own lots-O-tools. I'm a maintenance mechcanic by trade. I meant I had no tools with me when I looked at the truck.
My truck is a '72 C30 w/ 4:11 rear w/ leaf springs(OE).
The junk yard truck is an "82 C20 w/ leafs. It does not have the floating axles like mine. There's no SPID as the entire cab, dash, glove box, hump, tranny, & engine have been stripped.
Thanks for all the help just the same. I know how difficult it is trying to help online, especially w/o photos.

Flint70step · 04-29-2012, 04:53 PM

Pull the diff cover,mark one the teeth on the ring gear.Rotate and get a tooth count on the ring.Do the same for the pinion gear.Take your time and make sure you come up with the same numbers twice

then divide the number of ring teeth by the pinion tooth count and there is you rear end gear ratio.Its the long way around the barn to get over the fence but it works

hope this helps ya out

04-29-2012, 11:51 AM	#1
John Pasinski Registered User Join Date: Aug 2007 Location: orlando Posts: 456	Determine differential gear ratio I know this has been asked before, but can someone tell me how to identify the gear ratio on a posi (or semi posi) rear end? I"m asuming it's posi traction as there's a "raised" portion on the left side of the cover for a gear. I have not removed the cover as I had no tools at the time. It's in an '82 C20 deluxe with leafs. I'm trying to find a 3:73, 3:42 etc and swap out my 4:11. Please pardon my ignorance on this subject. __________________ A man's thoughts devise his ways; but it's GOD that directs his steps.

04-29-2012, 12:13 PM	#2
LockDoc The Older Generation Join Date: Aug 2006 Location: Montezuma, Iowa Posts: 25,426	Re: Determine differential gear ratio - There are several different ways. I'm not sure about the later ones but the '67-'72 have a code stamped on the pass side axle tube. You can also mark the tire and drive shaft with chalk and spin the tire and see how many revolutions the drive shaft makes. The best way is to remove the cover and check the numbers on the ring gear. That way you can check the posi situation and change the gear lube. I don't think the "raised portion" means anything. The later rear ends will not fit in our trucks without modifications. LockDoc __________________ Leon Locksmith, Specializing In Antique Trucks, Automobiles, & Motorcycles (My Dually Pickup Project Thread) http://67-72chevytrucks.com/vboard/s...d.php?t=829820 -

04-29-2012, 01:03 PM	#3
John Pasinski Registered User Join Date: Aug 2007 Location: orlando Posts: 456	Re: Determine differential gear ratio Thank you sir, I knew there are a couple of old timers on the board that could help me. Are you saying an '82 won't swap into my '72 w/o changes? Also, the way the truck is sitting, one wheel won't spin. So I am unable to do the "rotate the wheel & count" trick. Do all ring gears have numbers stamped into them? __________________ A man's thoughts devise his ways; but it's GOD that directs his steps.

04-29-2012, 01:10 PM	#4
too much stuff Roadtrip! Join Date: Feb 2003 Location: Grand Rapids MN Posts: 13,718	Re: Determine differential gear ratio If one wheel doesn't turn, it isn't posi. I don't think it is a direct swap either. Do you have coils? or leafs? How about the doner truck? And the "raised portion" won't tell you anything as they all had that. __________________ 67 gmc custom lwb dakota truck. Sold...And bought back 69 swb 4x4 project (in progress) 70 c-10 lwb texas truck 70 4x4/BB/bkts/posi/tach(retired) 70 swb (wifes) 70 jimmy camper 71 K-5 factory 2wd 71 cheyenne 4x4/tach/posi/ plow rig 71 Cheyenne20 BB, buckets, tach, tilt, 4 speed 72 jimmy CST wyoming rig 72 jimmy Colorado rig 72 lifted texas 4x4 3/4 ton burb 65 fury conv. 383/auto 65 chrysler 300 "L" 413 auto

04-29-2012, 02:27 PM	#5
special-K Special Order Join Date: Nov 2004 Location: Mt Airy, MD Posts: 85,861	Re: Determine differential gear ratio Does the truck have a SPID? Optional ratios are listed. Is this rear in an '82 C/20? It could have a full floating or a semi-floating 14-bolt 8-lug rear. Is your '72 a C/20? The shock mounts and spring pads will need relocated. Most likely,if it's and automatic it will have 3.73s. If I were you I would get some tools. At least a 1/2" and a 9/16" wrench. You need tools to own an old vehicle. __________________ "BUILDING A BETTER WAY TO SERVE THE USA"......67/72......"The New Breed" GMC '67 C1500 Wideside Super Custom SWB: 327/M22/3.42 posi.........."The '67" (project) GMC '72 K2500 Wideside Sierra Custom Camper: 350/TH350/4.10 Power-Lok..."The '72" (rolling) Tim "Don't call me a redneck. I'm a rough cut country gentleman" R.I.P. ~ East Side Low Life ~ El Jay ~ 72BLUZ ~ Fasteddie69 ~ Ron586 ~ 67ChevyRedneck ~ Grumpy Old Man ~

04-29-2012, 03:59 PM	#6
John Pasinski Registered User Join Date: Aug 2007 Location: orlando Posts: 456	Re: Determine differential gear ratio Special K......Pardon my grammar. Yes I own lots-O-tools. I'm a maintenance mechcanic by trade. I meant I had no tools with me when I looked at the truck. My truck is a '72 C30 w/ 4:11 rear w/ leaf springs(OE). The junk yard truck is an "82 C20 w/ leafs. It does not have the floating axles like mine. There's no SPID as the entire cab, dash, glove box, hump, tranny, & engine have been stripped. Thanks for all the help just the same. I know how difficult it is trying to help online, especially w/o photos. __________________ A man's thoughts devise his ways; but it's GOD that directs his steps.

04-29-2012, 04:53 PM	#7
Flint70step Registered User Join Date: Jan 2011 Location: Lake Travis,Texas Posts: 586	Re: Determine differential gear ratio Pull the diff cover,mark one the teeth on the ring gear.Rotate and get a tooth count on the ring.Do the same for the pinion gear.Take your time and make sure you come up with the same numbers twicethen divide the number of ring teeth by the pinion tooth count and there is you rear end gear ratio.Its the long way around the barn to get over the fence but it workshope this helps ya out __________________ 72 Cheyenne Super LWB 350/350 Factory A/C Tilt/Posi White/Med Red/White(on the way) 72 Custom Deluxe Burb(Arizona truck)Nice interior Great patina Project Patina Azul http://67-72chevytrucks.com/vboard/s...d.php?t=467668



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