How to determine rear end ratio? - The 1947 - Present Chevrolet & GMC Truck Message Board Network

chevybill · 10-03-2004, 08:19 AM

How can you determine the rear end ratio?

I marked the drive shaft and it turns twice to the rear wheel turning once, but that does not equate into a gear ratio?

Do I need to pull the rear end cover and look at how many times the ring gear turns in relation to the axle?

any help appreciated.

Did all lwb chevy and gmcs come with 2 piece driveshafts? I think my
uncle had a lwb gmc with out the carrier bearing in the driveshaft. I was talking to trans guy and he said lwb truck definitely needed a carrier bearing due to length. Do this sound correct?

da-burb · 10-03-2004, 08:44 AM

turn the wheel 2 full revolutions and count the drive shaft. Half of the wheels rotation is being split by the side gears. You will probably have a 4.11:1 ratio since you got 2 drive shaft turns in your first count. Just under 2 turns would be 3.73's.

72gord · 10-03-2004, 08:52 AM

If you only turned one wheel, you have to double the revolution count of the driveshaft to get an estimate of your ratio. One turn of the wheel and two turns of the driveshaft would be around a 4.10 ratio. If you are confident in your estimation, you can get a pretty good idea of the ratio. example: Mark the drive shaft yoke to the differential housing and the brake drum to the backing plate, rotate the wheel two turns slowly while counting the turns of the driveshaft. Two wheel revolutions will equal your ratio if you have an open diff. Make sure you only turn one wheel while the other is held in place. If it is a posi you will have to mark both drums and only turn the wheels one revolution. Just remember if you come up with 4 driveshaft turns and a little more you have to estimate if it is an extra .1 of a turn or .56 or what ever. By knowing the ratios offered in these trucks you can use an educated guess and get an accurate ratio. Most trucks had 3.73, 4.10 or 4.56 ratio. Hope this helps and is understandable. Good Luck, Gord

72gord · 10-03-2004, 08:53 AM

Guess burb said it in less words...same idea though

chevybill · 10-03-2004, 09:08 AM

thanks guys, on the right track just not enough turns.

Longhorn Man · 10-03-2004, 07:58 PM

popping the cover off and counting the gears is a good idea though, the oil in there is probably original...or at least 20 years old anyways.

10-03-2004, 08:19 AM	#1
chevybill Senior Member Join Date: Oct 2004 Location: calhoun city ms Posts: 940	How to determine rear end ratio? How can you determine the rear end ratio? I marked the drive shaft and it turns twice to the rear wheel turning once, but that does not equate into a gear ratio? Do I need to pull the rear end cover and look at how many times the ring gear turns in relation to the axle? any help appreciated. Did all lwb chevy and gmcs come with 2 piece driveshafts? I think my uncle had a lwb gmc with out the carrier bearing in the driveshaft. I was talking to trans guy and he said lwb truck definitely needed a carrier bearing due to length. Do this sound correct? __________________ ongoing 1970 c 10 lwb 1994 z-28 few mods.-owned since new 1978 vette silver/grey 25th anniv. paint

10-03-2004, 08:44 AM	#2
da-burb "Ochre Ogre" Join Date: Apr 2000 Location: Springfield, MN Posts: 3,558	turn the wheel 2 full revolutions and count the drive shaft. Half of the wheels rotation is being split by the side gears. You will probably have a 4.11:1 ratio since you got 2 drive shaft turns in your first count. Just under 2 turns would be 3.73's. __________________ 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

10-03-2004, 09:08 AM	#5
chevybill Senior Member Join Date: Oct 2004 Location: calhoun city ms Posts: 940	thanks guys, on the right track just not enough turns. __________________ ongoing 1970 c 10 lwb 1994 z-28 few mods.-owned since new 1978 vette silver/grey 25th anniv. paint

10-03-2004, 08:52 AM	#3
72gord Never enough time!! Join Date: Mar 2004 Location: sw Wisconsin Posts: 602	If you only turned one wheel, you have to double the revolution count of the driveshaft to get an estimate of your ratio. One turn of the wheel and two turns of the driveshaft would be around a 4.10 ratio. If you are confident in your estimation, you can get a pretty good idea of the ratio. example: Mark the drive shaft yoke to the differential housing and the brake drum to the backing plate, rotate the wheel two turns slowly while counting the turns of the driveshaft. Two wheel revolutions will equal your ratio if you have an open diff. Make sure you only turn one wheel while the other is held in place. If it is a posi you will have to mark both drums and only turn the wheels one revolution. Just remember if you come up with 4 driveshaft turns and a little more you have to estimate if it is an extra .1 of a turn or .56 or what ever. By knowing the ratios offered in these trucks you can use an educated guess and get an accurate ratio. Most trucks had 3.73, 4.10 or 4.56 ratio. Hope this helps and is understandable. Good Luck, Gord

10-03-2004, 08:53 AM	#4
72gord Never enough time!! Join Date: Mar 2004 Location: sw Wisconsin Posts: 602	Guess burb said it in less words...same idea though

10-03-2004, 07:58 PM	#6
Longhorn Man its all about the +6 inches Join Date: Jan 2001 Location: Hilliard Ohio Posts: 2,693	popping the cover off and counting the gears is a good idea though, the oil in there is probably original...or at least 20 years old anyways.



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