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

wxman1 · 10-14-2002, 08:20 PM

Someone mentioned a while back about how to check your rear
end ratio. So I jacked up one tire and a friend of mine at the office helped me out. He spun the tire while I watched the drive line. The tire rotated one while the drive shaft rotated twice. Of course that implies a 2.0 rear end. But...of course...that doesn't make sense. We did it several times and came up with the same numbers.

Anybody got any suggestions about what might be wrong? I'm really
puzzled. Talked to my brother who has 25+ years experience with Exxon
as a lubrication troubleshooter and he too is puzzled.

Jim M.
The weatherman

Zurbo · 10-14-2002, 08:30 PM

When you do this test with a one legger, (non posi) you will need to turn the lifted tire two revolutions. Count your driveline revs then. My estimate is that you are running 3.73's or 4.11's.

*Zkast* · 10-14-2002, 08:32 PM

with a non-posi rear end you'll need to jack up the center of the rear end.

Mark the drive shaft and BOTH tires. Rotate BOTH tires and have someone count the number of revolutions the drive shaft makes.

3 turns exactly will most likely be a 3:08

The reason your having trouble is, rotating only one wheel makes the driveshaft spin 1.5 times faster the speed if you have a non posi rear end. If you had a posi, you wouldnt be turning only one wheel, they would both turn equal amounts in the same direction

Longhorn Man · 10-14-2002, 11:04 PM

Did your brother recomend pullin the cover and chainging the oil?
You could count the teeth while your in there.

70c10 · 10-15-2002, 07:46 AM

I just did this on Sunday. I jacked the center up, put in gear and spun the passenger side tire 2 times. My driveshaft spun about 1.875 times. I already knew I had 3:73 gears but I am ordering a Posi soon and just wanted to be sure.

Blazer1970 · 10-15-2002, 07:58 AM

When you spin one tire on an open diff, turn it two times. The ratio is two to one, not 1.5.

wxman1 · 10-15-2002, 10:24 AM

Round 2 of the rear end ratio thing. OK...both wheels off the ground. Spin the tire and watch the drive shaft. When the driveshaft has made four revolutions the tire makes 2.2 revolutions...approximately. So then do I multiply by 2.0
or 1.5? after doing the division?

4.0/2.2 = 1.82

1.82 x 2.0 = 3.64

1.82 x 1.5 = 2.73

Bye the bye...when one tire is spun forward the other
tire remains static. When the same time is spun backward
the other tire spins the opposite direction. Other side tire
does the same thing. Is this normal?

Got the pressure washer out and sprayed down the chunk.
number of the passenger side of the chunk is:

3896889
NF

I think I remember that some body said that the first 3 numbers
on the ID was the ratio. Well...that all the gorey details. What
say ye masters of all things mechanical???

Jim M.
The weatherman

gmcnorton · 10-15-2002, 11:14 AM

IF YOU PULLED THE COVER THE RING IS STAMPED WITH THE NUMBER OF TEETH FOR EXAMPLE...43:13 DIVIDE THE SMALLER NUMBER INTO THE LARGER ONE 43 DIVIDED BY 13 =3.07 HOPE THAT HELPS

crazy longhorn · 10-15-2002, 01:25 PM

Gmc Norton's method is completely fool proof! It also, probably wouldnt hurt to change the lube if it hasnt been done recently. The spider gears are a 2 to 1 ratio if you want to do it that way. good luck....crazy AL

da-burb · 10-15-2002, 08:26 PM

wxman1, you only have to count 2 complete revolutions of the wheel while counting how many revolutions the drive shaft rotates. IF
2 turns of the wheel= 3 +a little extra drive shaft rotations= 3.07
2 turns of the wheel= 3-3/4 drive shaft rotations= 3.73
2 turns of the wheel= 4 +a little drive shaft rotations= 4.10

clear as mud?

10-14-2002, 08:20 PM	#1
wxman1 Senior Member Join Date: May 2002 Location: mt juliet, tennessee, usa Posts: 237	Rear End Ratio Someone mentioned a while back about how to check your rear end ratio. So I jacked up one tire and a friend of mine at the office helped me out. He spun the tire while I watched the drive line. The tire rotated one while the drive shaft rotated twice. Of course that implies a 2.0 rear end. But...of course...that doesn't make sense. We did it several times and came up with the same numbers. Anybody got any suggestions about what might be wrong? I'm really puzzled. Talked to my brother who has 25+ years experience with Exxon as a lubrication troubleshooter and he too is puzzled. Jim M. The weatherman

10-14-2002, 08:30 PM	#2
Zurbo Registered User Join Date: May 2001 Location: Bel Aire, Kansas, USA Posts: 186	When you do this test with a one legger, (non posi) you will need to turn the lifted tire two revolutions. Count your driveline revs then. My estimate is that you are running 3.73's or 4.11's. __________________ Zurbo's Chevy - 1972 Black/White Cheyenne Super. 402 Big Block /THD 400, factory air and tilt. My other Chevy - 1998 Camaro SS Last edited by Zurbo; 10-17-2002 at 02:54 AM.

10-15-2002, 07:46 AM	#5
70c10 67-72 Addict! Join Date: Feb 2002 Location: The TAX State! Posts: 7,857	I just did this on Sunday. I jacked the center up, put in gear and spun the passenger side tire 2 times. My driveshaft spun about 1.875 times. I already knew I had 3:73 gears but I am ordering a Posi soon and just wanted to be sure. __________________ Jim 1970 C/10 Fleetside w/Ghost Flames Lowered 4.5" front and 4" rear (Raked) 355/350 Turbo w/shift kit 10" Redneck Performance Verter w/2500 stall Hooker Super Comps part#2808-1 Performer RPM Air-Gap 12 Bolt w/3:73 gears- Eaton Posi Comp Cams XE262 with 1.6 Crane Energizers, Road Demon 625 and Brodix IK 180 heads

10-15-2002, 07:58 AM	#6
Blazer1970 Registered User Join Date: Dec 1999 Location: Old Mission, MI, USA Posts: 2,510	When you spin one tire on an open diff, turn it two times. The ratio is two to one, not 1.5. __________________ Tim

10-15-2002, 11:14 AM	#8
gmcnorton Registered User Join Date: Aug 2002 Location: Middleport, Pa. 17953 Posts: 737	RATIO IF YOU PULLED THE COVER THE RING IS STAMPED WITH THE NUMBER OF TEETH FOR EXAMPLE...43:13 DIVIDE THE SMALLER NUMBER INTO THE LARGER ONE 43 DIVIDED BY 13 =3.07 HOPE THAT HELPS __________________ 70 GMC SHORT STEP , 71 BLAZER ,72 JIMMY,72 BLAZER, JUST NEED THAT SUBURBAN NOW......WANTING MORE

10-14-2002, 08:32 PM	#3
Zkast Account Suspended Join Date: May 2002 Location: Olathe, Kansas Posts: 929	with a non-posi rear end you'll need to jack up the center of the rear end. Mark the drive shaft and BOTH tires. Rotate BOTH tires and have someone count the number of revolutions the drive shaft makes. 3 turns exactly will most likely be a 3:08 The reason your having trouble is, rotating only one wheel makes the driveshaft spin 1.5 times faster the speed if you have a non posi rear end. If you had a posi, you wouldnt be turning only one wheel, they would both turn equal amounts in the same direction

10-14-2002, 11:04 PM	#4
Longhorn Man its all about the +6 inches Join Date: Jan 2001 Location: Hilliard Ohio Posts: 2,693	Did your brother recomend pullin the cover and chainging the oil? You could count the teeth while your in there.

10-15-2002, 10:24 AM	#7
wxman1 Senior Member Join Date: May 2002 Location: mt juliet, tennessee, usa Posts: 237	Round 2 of the rear end ratio thing. OK...both wheels off the ground. Spin the tire and watch the drive shaft. When the driveshaft has made four revolutions the tire makes 2.2 revolutions...approximately. So then do I multiply by 2.0 or 1.5? after doing the division? 4.0/2.2 = 1.82 1.82 x 2.0 = 3.64 1.82 x 1.5 = 2.73 Bye the bye...when one tire is spun forward the other tire remains static. When the same time is spun backward the other tire spins the opposite direction. Other side tire does the same thing. Is this normal? Got the pressure washer out and sprayed down the chunk. number of the passenger side of the chunk is: 3896889 NF I think I remember that some body said that the first 3 numbers on the ID was the ratio. Well...that all the gorey details. What say ye masters of all things mechanical??? Jim M. The weatherman

10-15-2002, 01:25 PM	#9
crazy longhorn Fabricate till you "puke" Join Date: Jun 2002 Location: Ill Posts: 9,402	Gmc Norton's method is completely fool proof! It also, probably wouldnt hurt to change the lube if it hasnt been done recently. The spider gears are a 2 to 1 ratio if you want to do it that way. good luck....crazy AL

10-15-2002, 08:26 PM	#10
da-burb "Ochre Ogre" Join Date: Apr 2000 Location: Springfield, MN Posts: 3,558	wxman1, you only have to count 2 complete revolutions of the wheel while counting how many revolutions the drive shaft rotates. IF 2 turns of the wheel= 3 +a little extra drive shaft rotations= 3.07 2 turns of the wheel= 3-3/4 drive shaft rotations= 3.73 2 turns of the wheel= 4 +a little drive shaft rotations= 4.10 clear as mud? __________________ Bowtie Truck Stop Inc. Mid-West GM Truck Restoration Parts Supplier Your Key Parts, Auto Metal Direct, Dynacorn, and Goodmark dealer. like us @ www.facebook.com/BowtieTruckStop 1971 C-10 Suburban (Ochre) 1971 K-10 Suburban (Ochre) 1972 C-10 Suburban (Ochre) 1972 K-20 Suburban (Yellow- that just aint right!) Springfield, Minnesota 56087



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