Rear end gear size?

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68 CUDA FORMULA S

68 CUDA FORMULA S

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Is there a formula or a chart to tell what size gear is in the rear end of an 8 3/4 without pulling out the pig and reading what's stamped on it? it has been so long since I did something like this I just can't remember how it's done.
Thanks
Bob
 
If there isn't a tag you have to count the revolutions.,
 
66 CUDA said:
Is there a formula or a chart to tell what size gear is in the rear end of an 8 3/4 without pulling out the pig and reading what's stamped on it? it has been so long since I did something like this I just can't remember how it's done.
Thanks
Bob
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Pretty good chance it's a 3.23 ratio.
Is it stock or did you replace the 7 1/4 with the 8 3/4 ?
Guessing it's a V8
If you are running 2500 rpm at 65 mile good chance it is the 3.23
If you are running 3000 rpm at 65 mph, good chance it's a 3.55

If cruising at 65 mph and engine is not making much noise (not wound up tight) then good chance 2.76

Sure Grip or Open Differential makes a difference on the count of drive shaft to wheel rotation numbers.
 
It's goes something like keep on tire on the ground, then mark the drive shaft with chalk have someone rotate the tire one full turn count how many turns the drive shaft did then multiply it by 2. So if it did 1.75 turns times two will be 3.25 likely a 3.23 gear. But it's just a rough ball park
 
Slantsix64 said:
It's goes something like keep on tire on the ground, then mark the drive shaft with chalk have someone rotate the tire one full turn count how many turns the drive shaft did then multiply it by 2. So if it did 1.75 turns times two will be 3.25 likely a 3.23 gear. But it's just a rough ball park
Click to expand...
Unless it's a sure grip. LOL
 
I am talking about the rear end in this 68 Barracuda formula S 340 car that I picked up. There is no drive train in it yet and yes it is a sure grip. The fender tag states that it came from the factory with a 3.23 but this car was a drag car back in the day and there is no guarantees that the factory gear is still in there. Eventually the rear end will be worked on, but for right now I want to find out what gear is in there hopefully without having to pull it apart so I can get things rolling on a nice torque converter.
Bob
 
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66 CUDA said:
I am talking about the rear end in this 68 Barracuda formula S 340 car that I picked up. There is no drive train in it yet and yes it is a sure grip. The fender tag states that it came with a 3.23 but this car was a drag car back in the day and there is no guarantees that the factory gear is still in there. Eventually the rear end will be worked on, but for right now I want to find out what gear is in there hopefully without having to pull it apart so I can get things rolling on a nice torque converter.
Bob
Click to expand...
Start at post#2
 
66 CUDA said:
I want to find out what gear is in there hopefully without having to pull it apart so I can get things rolling on a nice torque converter.
Click to expand...

Put a chalk mark, piece of tape etc on one axle end and one on the yolk, I always use the right axle. Put a corresponding mark on the diff itself lined up with axle and yoke marks. Turn the axle 1 complete revolution while counting the number of revolutions of the yoke.

Post back the number.
 
There is no tag, and do I count how many revolutions of the prop shaft for one complete rotation of the wheel, and if so then is it like Slantsix64 added in post #4? My BUDhimers disease is catching up with me.
 
This only works for sure-grip, limited-slip, or positraction rear ends (both wheels rotate in same direction): Put the car on jack stands. Put the car in neutral. Put a chalk/tape mark on the driveshaft. Put a chalk/tape mark on the tire/wheel. Make sure you have a reference point for both marks. Go find an assistant who can count. Get under the car. Watch the mark on the driveshaft. Have the assistant rotate the wheel\tire some exact number of turns. The more the better. 4 is a good number. You count the revolutions of the driveshaft. Take that number and divide by the tire revolutions. Example: 4 tire revolutions with 14 1/4 revolutions of the driveshaft: 14.25÷4=3.56 gear. Since there was never a 3.56 in Mopars, it has to be a 3.55 gear.

If you have an open rear end (tires spin in opposite directions), you need to multiply your result by 2. In the above example, 4 tire revolutions with 7 1/8 driveshaft revolutions: 7.125÷4=1.781, times 2 = 3.56 gear.

The more turns you put on the tire the more precise will be the answer.
 
One revolution of the tire, then count the number of revolutions on the drive shaft.

3 and 1/3rd turns of the drive shaft would be a 3.23 ratio on the sure-grip.
 
Hey66Cuda,
Read YOUR POST #6You state that you are going to "WORK ON" the rear end latter. What does that mean? Change gear?
Well who ever sells you the converter will have a lot more questions than WHAT GEAR WAS IN THE CAR WHEN YOU GOT IT.
They will want motor specs, including cam, compression, maybe even intake manifold type, tire size, weight of car, and yes the rear end ratio.
That stuff sounds like it will be addressed "EVENTUALLY".
Put your mind on the converter then, not today.
 
Everything else is jotted down, the last piece of the puzzle is what size gear is in this car. If I find out that it was changed for drag racing and is something like in the 4's 4:13, 4:56, 4:88 then I will be wanting to change it to a 3:23 or like in my 66 a 3:55 or even a 3:91. Over the winter I would like to order my converter so I have it in the spring when I am ready to install the drive train. I am in the parts collection faze right now, and as far as "WORK ON" the rear end Donnie514 that means I am doing other things with this car right now and probably will not touch the rear end until next spring, change the fluid, make sure everything in side looks good, change the gearing if I have to, make sure it is road ready, that's where the "EVENTUALLY" comes in.
 
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George Jets said:
One revolution of the tire, then count the number of revolutions on the drive shaft.

3 and 1/3rd turns of the drive shaft would be a 3.23 ratio on the sure-grip.
Click to expand...
More like 3 1/4 turns. But why guess? Like I said above, more revolutions equal more accuracy.
 
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