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C1 checking the rear end ratio

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Old 02-16-2017, 02:53 PM   #1
68hemi
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Default C1 checking the rear end ratio

I am trying to check it on my 57. I have marked the drive shaft and turned the rear wheel but I only get about 4" past two turns on the wheel to one drive shaft turn. Which I calculate to like a 2.08 rear end which I have never heard of. Am I missing something here?
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Old 02-16-2017, 03:04 PM   #2
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You need to rotate the tire, and count how many times the drive shaft goes around, to determine the ratio.
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Old 02-16-2017, 03:13 PM   #3
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Easier way assuming your speedometer is fairly accurate and your tire is 27 inches diameter is at 2500 RPM in 4th gear yields the following

3.08= 65 Mph
3.36= 60 Mph
3.55= 57 Mph
3.70= 55 Mph
4.11= 49 Mph
4.56= 44 Mph
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Old 02-16-2017, 03:31 PM   #4
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What I said in the original post is supposed to be the correct way, right?
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Old 02-16-2017, 05:19 PM   #5
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Quote:
Originally Posted by 68hemi View Post
What I said in the original post is supposed to be the correct way, right?
Unless I misunderstand your post, I think you're doing it backwards.

To start with, do you know if you have an open rear or Positraction?

With an open rear, one tire has to be on the ground. Turn the other tire two complete rotations, and count how many times the drive shaft rotates. If the driveshaft makes slightly more than 4 full turns, you have a 4:11 rear.

If you have a Posi, both tires have to be off the ground, and then you turn the tire only one rotation. Count the number of times the drive shaft turns, to determine the axle ratio.
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Old 02-16-2017, 07:18 PM   #6
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I am not sure if it is posi or not but with both wheels off the ground you have to block one wheel to get the drive shaft to turn. I did this and turned the other wheel about 4" past two revolutions and the drive shaft turned one revolution.
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Old 02-16-2017, 10:56 PM   #7
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If it's a Corvette rear end, then the easiest way is to get the code stamped on the differential and then look it up. If it's a posi, it'll have a large "P" on the drivers side.
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Old 02-17-2017, 09:35 AM   #8
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First, do you have a posi or limited slip rear? If you're not sure, just jack up the rear so both tires are off the ground and put your jackstands under the axle. Now turn one of the tires. If the other tire turns the same way or doesn't turn at all, you should have a posi or limited slip. If the tire turns opposite of the way the first tire spins, then it's probably an open rear end.

If you have a car with a posi or limited slip rear, jack up the car and get both rear tires off the ground. Now put your jackstands under the axle. Make a line on the pinion yoke and onto the rear end with chalk or a grease pencil. Do the same to the tire on the car, so you can see when the tire has rotated exactly one turn.

Now count the number of turns of the driveshaft that it takes (use your chalk marks) to make the rear tire turn one complete rotation. (Transmission needs to be in nuetral or out of gear.)

If your car has an open rear, jack up only one rear tire and leave the other on the ground. Now put a jackstand under the lifted side. Turn the tire and again count the number of turns of the driveshaft, but this time you must turn the tire two full rotations.

So:

Posi: Jack both wheels off the ground. Turn one wheel 1 complete turn.

- Count the turns on the drive shaft.

Non-posi: Jack one wheel off the ground. Turn the wheel 2 complete turns.

- Count the turns on the drive shaft.

If it takes about 2 and three-quarters turns, it's around a 2.73 gear ratio. If it's a little more than 3, you have 3.08s. If it takes about 3 and a quarter turns of the driveshaft, you have 3.23 or 3.25 gears. Very close to 3 and three-quarter turns, 3.70 or 3.73 gears. This method won't work very well on a posi that's worn-out and won't spin both tires equally.
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Old 02-17-2017, 05:14 PM   #9
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As I have said, I have followed the procedures in this thread and I get 2 complete revolutions of the tire plus about 4" to one revolution of the drive shaft. This would indicate to me about a 2.08 ratio and I have never heard of anything higher than about a 2.76 even in a passenger car??????
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Old 02-17-2017, 05:36 PM   #10
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The drive shaft needs to turn several times/wheel rev, not once. 3.5:1 ratio = 3.5 DS turns to 1 tire/wheel turn, if both tires are turning equally, 3.5 DS turns = 2 tires turns if one tire stationary.

No way will two complete revs of tire generate only one turn of the drive shaft. Unless something is broken inside, like an axle.

Try it with the other tire raised and the first one on the ground.

Doug
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Old 02-17-2017, 05:44 PM   #11
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Or maybe all your spider gears are stripped, or your pinon shaft is broken.

Have you actually driven the car, or just pushed it around?

Doug
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Old 02-17-2017, 05:57 PM   #12
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Quote:
Originally Posted by AZDoug View Post
Or maybe all your spider gears are stripped, or your pinon shaft is broken.

Have you actually driven the car, or just pushed it around?

Doug

Oh no, I have been driving it.
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Old 02-17-2017, 05:59 PM   #13
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Quote:
Originally Posted by AZDoug View Post
The drive shaft needs to turn several times/wheel rev, not once. 3.5:1 ratio = 3.5 DS turns to 1 tire/wheel turn, if both tires are turning equally, 3.5 DS turns = 2 tires turns if one tire stationary.

No way will two complete revs of tire generate only one turn of the drive shaft. Unless something is broken inside, like an axle.

Try it with the other tire raised and the first one on the ground.

Doug

Doug I sent you an email with my phone number, I live in Cottonwood.
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Old 02-18-2017, 06:18 AM   #14
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Quote:
Originally Posted by 68hemi View Post
I get 2 complete revolutions of the tire plus about 4" to one revolution of the drive shaft.
2 complete revolutions of the tire = about 4 revolutions of the driveshaft.

With a single side rear end, the ACTUAL revolutions of the drive shaft after TWO complete turns of one tire (with the other tire held stationary) is the rear ratio.

You say "about" 4 turns of the driveshaft. Is it more like 3 3/4 turns, or just slightly more than 4 turns?

If 3 3/4 turns - 3.73 open end rear ratio.

if slightly more than 4 turns - 4.11 open end rear ratio, or a badly worn posi.

I would say you have a 4.11 single side rear gear ratio.

An "open" differential has a lot to do with the other tire being part of the equation. Let's use an example of a 3 to 1 rear gear ratio, and lets look at it from the perspective of the driveshaft/pinion. When you're going in a straight line, three complete revolutions of the driveshaft will turn BOTH wheels one revolution. When one wheel is locked, ANY differential will basically multiply the rotation of the other wheel by a factor of two. Even a posi-type differential will do this, it just takes some force to overcome the friction of the clutches in the posi unit.

Last edited by jv04; 02-18-2017 at 06:35 AM.
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Old 02-18-2017, 10:48 AM   #15
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Quote:
Originally Posted by jv04 View Post
2 complete revolutions of the tire = about 4 revolutions of the driveshaft.

With a single side rear end, the ACTUAL revolutions of the drive shaft after TWO complete turns of one tire (with the other tire held stationary) is the rear ratio.

You say "about" 4 turns of the driveshaft. Is it more like 3 3/4 turns, or just slightly more than 4 turns?.
No, he say the drive shaft turns ONCE per two revolutions of one tire.

I am going to call him later this morning.

Doug
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