[chevelleshack]

Hello fellas , quick question on 12 bolt rear axle . I believe the original 3:31 gear in my ls-5 chevelle has been changed . There are no gm stampings on the ring gear plus it cruises at 60 mph about 3500 rpm , seems a little high to me . Anyway , would like to put an original gm gear back in it & was wondering if there is any difference in the 454 12 bolt gearset vs the 396 version ? Or is it just the yoke that is different between the two ? Also , anyone have a gearset they would like to get rid of ? Prefer nov or dec 69 or jan 70 dates if possible. Thanks !

 

[zookpr]

Hello fellas , quick question on 12 bolt rear axle . I believe the original 3:31 gear in my ls-5 chevelle has been changed . There are no gm stampings on the ring gear plus it cruises at 60 mph about 3500 rpm , seems a little high to me . Anyway , would like to put an original gm gear back in it & was wondering if there is any difference in the 454 12 bolt gearset vs the 396 version ? Or is it just the yoke that is different between the two ? Also , anyone have a gearset they would like to get rid of ? Prefer nov or dec 69 or jan 70 dates if possible. Thanks !

No difference in the passenger car gears except ratio and also application may have a different yoke. 454 should be a larger yoke - no worries there since you have it. All you need is the gear set and any worn parts seals, etc..

 

[chevelleshack]

That was what i was hoping for . Now to find a gm gearset !

 

[Kirk's67SS]

What is the Diameter of your Rear Tire ?

If I use a 26" Diameter Tire in the Calculator I used
and your Speedo is Correct for the Rearend Gear Ratio then your RearEnd Ratio = 4.56
Engine RPM Calculator Off-Road and 4X4 Calculators and Guides

Axle Gear Ratio:4.56 : 1 Tire Diameter:26 Inches	Vehicle Speed:60 MPH Transmission Ratio:1.00 : 1
Engine Speed:3536 RPM

 

[big gear head]

Some of the 3.31 gears are marked on the mating surface of the gear where it bolts to the differential and you can't see the numbers unless you remove the gear. Are there any markings on the gear and did you look at the end of the pinion gear for markings? Post a picture of the ring gear and post it here. If it has a different gear ratio then they may have used a ring gear spacer.

 

[72soft-top]

Deleted since others beat me to it.

 

[billdini]

My 3.31 with 235/60/15's run at 2600 at 60 mph. My stock gears are clearly marked on the ring. My 3.73's ran at 3000 and usually 4.11 + are in your range depending on tire size.

 

[chevelleshack]

I don't think it as deep as a 4.56 so 3500 may be a little off the mark , 32, 3300 rpm maybe ? Doesn't have a spacer so probably at the most i would think 3.73 unless the carrier is replaced ( hope not ). But it does have an external speedo thingy ( right at trans & cable threads to it , can't think of proper term right now ) . Wouldn't think it needed that with factory gear & speedo is correct so somebody has been messing with it . Car is running driving right now & won't be able to bust into it for a few weeks but i do remember no numbers on the outside edge of ring but didn't think to look at end of pinion . Thanks for the info so far !

 

[Leadhead]

What is the Diameter of your Rear Tire ?

If I use a 26" Diameter Tire in the Calculator I used
and your Speedo is Correct for the Rearend Gear Ratio then your RearEnd Ratio = 4.56
Engine RPM Calculator Off-Road and 4X4 Calculators and Guides

Axle Gear Ratio:4.56 : 1 Tire Diameter:26 Inches	Vehicle Speed:60 MPH Transmission Ratio:1.00 : 1
Engine Speed:3536 RPM

Thanks for this!!

 

[Lew540]

To double check your yoke look at the Spicer chart you should have a 1330 series U Joint back there. There are two ways to tell I think (and check the spicer chart) the first is the caps should be 1 1/8 inch in diameter and I think they are 3.625 inches across while the smaller 1310 series are 1 1/16 inch caps and I think the cross is smaller at like 3.25 inches across.

 

[Kirk's67SS]

I don't think it as deep as a 4.56 so 3500 may be a little off the mark , 32, 3300 rpm maybe ? Doesn't have a spacer so probably at the most i would think 3.73 unless the carrier is replaced ( hope not ). But it does have an external speedo thingy ( right at trans & cable threads to it , can't think of proper term right now ) . Wouldn't think it needed that with factory gear & speedo is correct so somebody has been messing with it . Car is running driving right now & won't be able to bust into it for a few weeks but i do remember no numbers on the outside edge of ring but didn't think to look at end of pinion . Thanks for the info so far !

Again it is dependent on the Size/Diameter of your Rear Tire ??

Engine RPM Calculator Off-Road and 4X4 Calculators and Guides
Enter your axle gear ratio, tire diameter, vehicle speed, and transmission gear ratio in the blue fields and click the calculate button to find your engine RPM.

Axle Gear Ratio: 4.10 : 1 Tire Diameter: 26 Inches	Vehicle Speed: 60 MPH Transmission Ratio: 1 : 1
Engine Speed: 3,179 RPM

 

[Kirk's67SS]

My 67 with a S60 RearEnd and 3:70 Gears with a TH400 1 to 1 Ratio in 3rd AND 27.8" Diameter High Tires

Engine RPM Calculator
Enter your axle gear ratio, tire diameter, vehicle speed, and transmission gear ratio in the blue fields and click the calculate button to find your engine RPM.

Axle Gear Ratio: 3.70 : 1 Tire Diameter: 27.8 Inches	Vehicle Speed: 60 MPH Transmission Ratio: 1 : 1
Engine Speed: 2,683 RPM

 

[Tod74]

You don’t have to have a spacer to be 4.10 or 4.56. They make gears specifically fit the 3 series carrier. They are just thicker

 

[chevelleshack]

true...... forgot about that

 

[Bob Tiley]

Just count the teeth on the ring and divide by the teeth on the pinion. Or just jack up the rear end and turn the tires until you get one rotation on the driveshaft and count how many turns of the tire.

 

[1971-Chevelle]

Just count the teeth on the ring and divide by the teeth on the pinion. Or just jack up the rear end and turn the tires until you get one rotation on the driveshaft and count how many turns of the tire.

That is backwards. You count how many times the driveshaft turns for one revolution of the tire(s).

HOW TO: Determine the Rear Axle Ratio you have - Modern Driveline

Knowing your rear end’s gear ratio is crucial when you are considering a transmission upgrade

www.moderndriveline.com

 

[Jack454]

My 3.31 with 245/60-15's @ 3000rpm... 70mph I believe...'71 454 Chevelle with 12 bolt

 

[Aml383]

I know this song by heart lol
Rpm = (mph x rear dif x 336 ) / tire diameter
X final gear or whatever gear you want to calculate.

If you're 3500 at 60 mph with the stock 14 rim and stock tire (I've got one on a 66 chevelle sitting right here ) its about a 24.25 tire diameter.
Then you've got 4:10 gears .
But that doesnt mean at all that that's the gear that came in the car out of the box in 64 .

Calculating using The size of the rim/ tire along with speedometer / tachometer readings that is currently on the car will tell you nothing about the stock differential that was/is in that car from the factory nor will counting revolutions of the drive shaft versus tire revolution UNLESS it is the stock size rim and tire on the car and UNLESS the original factory differential is in fact in the car .
The only thing you'll learn by using the formula I've given you here is what the differential is TODAY and thats if accurate input is applied into the formula but that will tell you nothing about original gears.

Even if there are markings, on the rear end that you can find in gm data that correlate to a factory gear ratio that still doesn't tell you what gears are actually in the rear end.
The car is 55 years old .
Take the cover off and confirm what gears are actually in the car today and then do your homework to find out what cane in that car stock if its that important to you

 

[Second_chance_SS]

Some of the 3.31 gears are marked on the mating surface of the gear where it bolts to the differential and you can't see the numbers unless you remove the gear. Are there any markings on the gear and did you look at the end of the pinion gear for markings? Post a picture of the ring gear and post it here. If it has a different gear ratio then they may have used a ring gear spacer.

Dang, Freddie, that is one sad looking avatar...

 

[big gear head]

Yeah, that was my last Chevelle. It got parted out. I probably need to use a picture of my first Chevelle.