Rear End Ratio ID - The 1947 - Present Chevrolet & GMC Truck Message Board Network

BeBeep · 04-22-2007, 06:04 PM

Hi Folks,

I have a 79 GMC Heavy Half and I am trying to ID the rear end ratio and if it is a limited slip or not. The options list is not in the glovebox and I cannot seem to find the ratio etc anywhere on the truck. Is there someplace I can find the ratio and if it is limited slip or did a certain ratio combo come on this truck? The axle is a 12 bolt 8 3/4 unit, that I know for sure.

I need to change the fluid and I need to see if I need the aditive or not.

Please advise and thank you.

msgross · 04-22-2007, 06:30 PM

well you have a few options...

1. pop the cover and read the numbers on the ring gear... 41/11 = 3.73 and so on... If it has springs in the carrier then you have limited slip, if not then it's open. You will see the spings that i'm talking about...
2. Jack up one wheel (rear preferred), if you can roll it then it's open. Mark the drive shaft and see how many times the driveshaft turns to roll the wheel twice, (i.e 3 times = [3.08's? 3 3/4 then 3/73?) If the wheel doesnt turn then it's limited slip...
3. In that case jack up both wheels and do the same as above but only count how many times the driveshaft spins vs. one wheel revolution?
4. Hold the brake and do a burnout, 1 black mark = open 2 marks = limited slip

If you have another vehicle to run to Autozone then just pull the cover and drain it...

Make since? I'm lost now...

BeBeep · 04-22-2007, 06:35 PM

Thank you for the info, since I have to remove the cover anyway I will go that route.

Thanks.

msgross · 04-22-2007, 06:36 PM

Quote:

Originally Posted by BeBeep

Thank you for the info, since I have to remove the cover anyway I will go that route.

Thanks.

Just be sure that you have a way to travel and get more.... Wouldn't be the first time someone stranded themselves (I did it with the wrong oil filter once!)

BeBeep · 04-22-2007, 06:54 PM

Thanks but no worries, I have a spare vehicle just incase.

jimfulco · 05-01-2007, 03:46 AM

You may have 4 sets of numbers on the edge of the ring gear. One pair is the tooth counts, and one pair is the date of manufacture.

I don't know which pair comes first, but the date code should be obvious.

04-22-2007, 06:04 PM	#1
BeBeep Registered User Join Date: Mar 2007 Location: Ft. Worth, TX Posts: 20	Rear End Ratio ID Hi Folks, I have a 79 GMC Heavy Half and I am trying to ID the rear end ratio and if it is a limited slip or not. The options list is not in the glovebox and I cannot seem to find the ratio etc anywhere on the truck. Is there someplace I can find the ratio and if it is limited slip or did a certain ratio combo come on this truck? The axle is a 12 bolt 8 3/4 unit, that I know for sure. I need to change the fluid and I need to see if I need the aditive or not. Please advise and thank you.

04-22-2007, 06:30 PM	#2
msgross Registered User Join Date: Jul 2006 Location: Central PA Posts: 12,201	Re: Rear End Ratio ID well you have a few options... 1. pop the cover and read the numbers on the ring gear... 41/11 = 3.73 and so on... If it has springs in the carrier then you have limited slip, if not then it's open. You will see the spings that i'm talking about... 2. Jack up one wheel (rear preferred), if you can roll it then it's open. Mark the drive shaft and see how many times the driveshaft turns to roll the wheel twice, (i.e 3 times = [3.08's? 3 3/4 then 3/73?) If the wheel doesnt turn then it's limited slip... 3. In that case jack up both wheels and do the same as above but only count how many times the driveshaft spins vs. one wheel revolution? 4. Hold the brake and do a burnout, 1 black mark = open 2 marks = limited slip If you have another vehicle to run to Autozone then just pull the cover and drain it... Make since? I'm lost now... __________________ The Garage: 1968 K-10 SWB - "Project Money Pit" 1996 Z-71 - "huntin rig" 1969 C-10 LWB (SOLD) "Project flip that truck or else" 1993 Passport, F@rd 1-ton (SOLD)"Project Cousin Eddie" My Garage Build "The 1,000 footer" Last edited by msgross; 04-22-2007 at 06:34 PM.

04-22-2007, 06:35 PM	#3
BeBeep Registered User Join Date: Mar 2007 Location: Ft. Worth, TX Posts: 20	Re: Rear End Ratio ID Thank you for the info, since I have to remove the cover anyway I will go that route. Thanks.

04-22-2007, 06:54 PM	#5
BeBeep Registered User Join Date: Mar 2007 Location: Ft. Worth, TX Posts: 20	Re: Rear End Ratio ID Thanks but no worries, I have a spare vehicle just incase.

05-01-2007, 03:46 AM	#6
jimfulco Registered User Join Date: May 2003 Location: Shreveport LA Posts: 3,170	Re: Rear End Ratio ID You may have 4 sets of numbers on the edge of the ring gear. One pair is the tooth counts, and one pair is the date of manufacture. I don't know which pair comes first, but the date code should be obvious.



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