What rear end do I have and what gear ratio? - The 1947 - Present Chevrolet & GMC Truck Message Board Network

nathank · 03-31-2005, 12:05 AM

How do you tell what rear end you might have? I have always been a Mustang guy so these Chevys are new to me.

Also, I always thought gear ratio was determined by how many revolutions of the driveshaft for one revolution of the rear wheel... but don't even count 2 by the time the wheel is all the way around.

67shortie · 03-31-2005, 04:01 AM

The only wa to tell for sure is to take the rear cover off and count the teeth on the ring and pinion and devide. Another way to do is take your speed and rpm, go to a site that has a calculator for tire size, speed and rpm and it computes your gear ratio for you. one off hand is www.smokemup.com

That's the only one I have used in the past.

Longhorn Man · 03-31-2005, 06:07 AM

You probably have a 12 bolt with a 7.73. You need to spin the tire twice since the other one didn't move.

Longhorn Man · 03-31-2005, 06:08 AM

But I agree 100% with draining the oil...it is probably only doin 50% of it's work now from old age.

Tx Firefighter · 03-31-2005, 08:11 AM

You have to jack up a wheel and count the driveshaft revolutions while turning the wheel 2 times around.

The result will be your ratio.

Putter · 03-31-2005, 09:43 AM

And I am pretty sure that Longhorn Man meant to say 3.73....not 7.73. That would be one hell'of'a ratio.

nathank · 03-31-2005, 10:17 AM

Quote:

Originally Posted by 67shortie

The only wa to tell for sure is to take the rear cover off and count the teeth on the ring and pinion and devide. Another way to do is take your speed and rpm, go to a site that has a calculator for tire size, speed and rpm and it computes your gear ratio for you. one off hand is www.smokemup.com

That's the only one I have used in the past.

Well, that brings us to another hurdle.. I can tell the speedo is off a little.
Do you change the speedo gears at the tranny like you do on the Mustangs to correct the speedo?

When I open the rear cover and count the teeth, what do I need to divide by to find the ratio?

shifty · 03-31-2005, 01:34 PM

Thanks for asking this, nathan. I've been wanting to ask the same ...

**JIMs70GMC** · 03-31-2005, 05:00 PM

ring gear divided by pinion equals ratio.

Longhorn Man · 03-31-2005, 10:55 PM

yes, I did in fact mean 3.73...good eye.

jimfulco · 04-01-2005, 01:49 AM

But you gotta make sure the other wheel didn't move while you weren't looking at it.

For speedo calibration, go to www.novaresource.org/speedo.htm

And the Novaresource info also works fairly well on Fords if you can find the right gears.

Ozark Tim · 04-01-2005, 11:13 AM

Alright you guys are killing me! The axle code is on the axle tube at the top between the differential case, and the trailing arm or leaf spring mount ( whichever you have ) on the passenger side if a 1/2 ton. Clean it off, get a flashlight and write them down. Then pm me the code and I will tell you what it has without the greasy mess. Also if the transmission/axle are the originals you can purchase the correct plastic speedo gear based on the ratio of the rear end. You have to wonder why it is off though. Might have had a tranny change by someone without concern. Also the tire height factors in on the overall ratio. If you increase the tire height from say a original size, oh say G78-15 tire which is roughly 27 inches high to a 33 inch tall tire you have changed your overall axle ratio and therefore the gear in the tranny is not a match any longer. At that point there are multipliers that can be aquired from a speedo shop if you can find one any more. If you have the case open already you do not have to count teeth. The ring gear has the numerals ( teeth count on it ) 41:11 would be 41teeth on the ring divided by 11 teeth on the pinion. Do the math and you have 3:73. Like I said if you want to pm me the code I will be happy to help.
Tim

Ozark Tim · 04-01-2005, 12:35 PM

I thought I better give you a sample and ask you do you have the protecto plate? If you do the axle code is on it and you wont even have to crawl under the truck. If not you are looking for something on the top or front of the axle tube on the passenger side between the differential case and spring mount or trailing arm mount like my sample below
Sample from one of my 72 chevy 1/2 ton's------RHA W33401 E
The RHA represents the ratio. In this case it is 3:07
The W represents the plant the axle was built. In this case Warren.
The 334 represents the day of the year ( there is 365 ). In this case the 334th day of the year is Dec 7th ( Which by the way will normally be within 5 days of final assembly of the truck ) This truck was assembled 12-13-1971.
The 01 represents the shift the axle was produced. In this case the 1st shift
If this sequence of numbers is followed by an E, W, or D it had a positraction by either Eaton, Warner, or Dana. In this case an Eaton.
This is just a sample and there are other codes for 20, and 30 series.
Also for example for 72 a 3:07 ratio can be a RHA, RHB, RHS, RHT, RRR, RRS, RRT, or RRW.
Fortunately I have all the codes if you would like to PM me with your code & vin # so I know the year and series truck.
Tim

nathank · 04-01-2005, 03:44 PM

Thanks to everyone for the input on this. Tim, I will look at the code, but there's no telling if the gears in the truck are original or might have been swapped.

Longhorn Man · 04-01-2005, 09:42 PM

Axle code on protecto plate?

My protecto plate only has the original owner's name and some other info...nothing tech related though.

03-31-2005, 12:05 AM	#1
nathank resident geek Join Date: Jan 2005 Location: Dusty West Texas Posts: 77	What rear end do I have and what gear ratio? How do you tell what rear end you might have? I have always been a Mustang guy so these Chevys are new to me. Also, I always thought gear ratio was determined by how many revolutions of the driveshaft for one revolution of the rear wheel... but don't even count 2 by the time the wheel is all the way around. __________________ 71 C-10 stepper 88 Moostang T-Top w/ lots of goodies

03-31-2005, 04:01 AM	#2
67shortie Green club Join Date: May 2002 Location: Central CA Posts: 1,059	The only wa to tell for sure is to take the rear cover off and count the teeth on the ring and pinion and devide. Another way to do is take your speed and rpm, go to a site that has a calculator for tire size, speed and rpm and it computes your gear ratio for you. one off hand is www.smokemup.com That's the only one I have used in the past. __________________ 71 short/fleet ECE 4/6 drop, LQ4/700r4/3.42 gears 70 SS396 Chevelle 427/200-4R/3.73posi 61 Lincoln Continental, black on black Son you are going to drive me to drinkin if you don't stop drivin that hot rod lincoln

03-31-2005, 08:11 AM	#5
Tx Firefighter Watch out for your cornhole ! Join Date: Jun 2003 Location: Azle, Texas Posts: 14,162	You have to jack up a wheel and count the driveshaft revolutions while turning the wheel 2 times around. The result will be your ratio. __________________ I'm on the Instagram- @Gearhead_Kevin

03-31-2005, 09:43 AM	#6
Putter More Cowbell.... Join Date: Mar 2004 Location: Puckett, MS Posts: 3,569	And I am pretty sure that Longhorn Man meant to say 3.73....not 7.73. That would be one hell'of'a ratio. __________________ *There once was a member from Puckett.....Who ( fill in the blank blank blank* ) bucket**

03-31-2005, 01:34 PM	#8
shifty Questionable Join Date: Mar 2005 Location: Atlanta Posts: 13,376	Thanks for asking this, nathan. I've been wanting to ask the same ... __________________ If I've got anything up for grabs, it'll be here: 7-hole gauge cluster for a 67-72 p/u FREE (link) I can't check the forum daily. If I don't reply to you within 24 hours, drop me a PM! I'm (hopefully) still alive and will reply faster to a PM.

03-31-2005, 06:07 AM	#3
Longhorn Man its all about the +6 inches Join Date: Jan 2001 Location: Hilliard Ohio Posts: 2,693	You probably have a 12 bolt with a 7.73. You need to spin the tire twice since the other one didn't move.

03-31-2005, 06:08 AM	#4
Longhorn Man its all about the +6 inches Join Date: Jan 2001 Location: Hilliard Ohio Posts: 2,693	But I agree 100% with draining the oil...it is probably only doin 50% of it's work now from old age.

03-31-2005, 05:00 PM	#9
JIMs70GMC user # 2756 Join Date: Jun 2001 Location: Chesapeake, Virginia Posts: 4,612	ring gear divided by pinion equals ratio. __________________ 1970 K25, 8' stepside bed 350/465/205 44 up front, 60 in the rear 4.10s rolling on 33" Dunlop MTs 1986 K5, 350/465/208 Dana 60/14 bolt from a cucv 36" Super Swampers TSL/SX 1983 K20 w/ CUCV axles, 350/700R4/208 sitting on 37" Goodyears 1986 M1031 6.2 diesel, TH400/NP205 locker in the rear and a LS in the front, all stock for now..... 1986 K30, 350/400/205 dana 60 and 14 bolt. I kept the drivetrain. Body/bad and chassis are gone. 1981 K30, 350/465/205 dana 60 and dually 14 bolt. Has a G80, and a flat bed. Going to replace the flat bed. 1985 K20, 350/400/208 10 bolt and SF 14 bolt. I wonder where I can find some 1 tons. Hmmmmm

03-31-2005, 10:55 PM	#10
Longhorn Man its all about the +6 inches Join Date: Jan 2001 Location: Hilliard Ohio Posts: 2,693	yes, I did in fact mean 3.73...good eye.

04-01-2005, 01:49 AM	#11
jimfulco Registered User Join Date: May 2003 Location: Shreveport LA Posts: 3,170	But you gotta make sure the other wheel didn't move while you weren't looking at it. For speedo calibration, go to www.novaresource.org/speedo.htm And the Novaresource info also works fairly well on Fords if you can find the right gears. Last edited by jimfulco; 04-01-2005 at 01:51 AM.

04-01-2005, 11:13 AM	#12
Ozark Tim Registered User Join Date: Aug 2004 Location: Oakville, MO Posts: 1,018	Alright you guys are killing me! The axle code is on the axle tube at the top between the differential case, and the trailing arm or leaf spring mount ( whichever you have ) on the passenger side if a 1/2 ton. Clean it off, get a flashlight and write them down. Then pm me the code and I will tell you what it has without the greasy mess. Also if the transmission/axle are the originals you can purchase the correct plastic speedo gear based on the ratio of the rear end. You have to wonder why it is off though. Might have had a tranny change by someone without concern. Also the tire height factors in on the overall ratio. If you increase the tire height from say a original size, oh say G78-15 tire which is roughly 27 inches high to a 33 inch tall tire you have changed your overall axle ratio and therefore the gear in the tranny is not a match any longer. At that point there are multipliers that can be aquired from a speedo shop if you can find one any more. If you have the case open already you do not have to count teeth. The ring gear has the numerals ( teeth count on it ) 41:11 would be 41teeth on the ring divided by 11 teeth on the pinion. Do the math and you have 3:73. Like I said if you want to pm me the code I will be happy to help. Tim

04-01-2005, 12:35 PM	#13
Ozark Tim Registered User Join Date: Aug 2004 Location: Oakville, MO Posts: 1,018	I thought I better give you a sample and ask you do you have the protecto plate? If you do the axle code is on it and you wont even have to crawl under the truck. If not you are looking for something on the top or front of the axle tube on the passenger side between the differential case and spring mount or trailing arm mount like my sample below Sample from one of my 72 chevy 1/2 ton's------RHA W33401 E The RHA represents the ratio. In this case it is 3:07 The W represents the plant the axle was built. In this case Warren. The 334 represents the day of the year ( there is 365 ). In this case the 334th day of the year is Dec 7th ( Which by the way will normally be within 5 days of final assembly of the truck ) This truck was assembled 12-13-1971. The 01 represents the shift the axle was produced. In this case the 1st shift If this sequence of numbers is followed by an E, W, or D it had a positraction by either Eaton, Warner, or Dana. In this case an Eaton. This is just a sample and there are other codes for 20, and 30 series. Also for example for 72 a 3:07 ratio can be a RHA, RHB, RHS, RHT, RRR, RRS, RRT, or RRW. Fortunately I have all the codes if you would like to PM me with your code & vin # so I know the year and series truck. Tim

04-01-2005, 03:44 PM	#14
nathank resident geek Join Date: Jan 2005 Location: Dusty West Texas Posts: 77	Thanks to everyone for the input on this. Tim, I will look at the code, but there's no telling if the gears in the truck are original or might have been swapped. __________________ 71 C-10 stepper 88 Moostang T-Top w/ lots of goodies

04-01-2005, 09:42 PM	#15
Longhorn Man its all about the +6 inches Join Date: Jan 2001 Location: Hilliard Ohio Posts: 2,693	Axle code on protecto plate? My protecto plate only has the original owner's name and some other info...nothing tech related though.

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