Determinig rear end gearing

[vegaschevy]

I do not have any tags on the diff cover or anything. How can I figure out the gearing on the rear end? count the teeth? any ideas?

[1968SWBBigBlock]

If you pop the cover off and spin the rearend around you should find the tooth count on the ring gear for the ring and pinion

[ERASER5]

Why does everyone go straight for the messiest way to determine gear ratio?

Gear ration and limited slip can be determined easily and if you have a creeper, only your knees will get dirty.

Determine if you have an open differential or a limited slip differential.
1. Put the transmission in neutral.
2. Jack up the driver’s side rear wheel. Use all safety precautions and jack stands.
3. Try to turn the tire.
4. If tire turns, you have an open differential. Go to Check Gear Ratio with Open Diff.
5. If tire does not turn, you have a limited slip differential. Go to Check Gear Ratio with a LSD.

Gear Ratio with Open Diff
1. With the driver’s side tire still in the air (and jack stands in place), rotate the tire until the valve stem is at 12:00.
2. Roll under the truck and chalk mark the yoke and differential case at the 3:00 or any where you will be able to see it from the tire.
3. Now roll back to the tire and while watching the chalk mark, rotate the tire exactly 2 full rotations. Use the valve stem that was at 12:00. Count the full rotations of the drive shaft.
4. Now, the valve stem is back at 12:00, but the chalk mark is not at 3:00. Roll under the truck and determine the partial turn. It could a half turn, it could be a three-quarter turn, or maybe a tenth of a turn.
5. Add the full turns to the fraction of a turn and you have you ratio. 3-3/4 is 3.75:1.
6. There are only a few actual gear ratios and this will get you within a few hundredths.
The reason for the 2 tire rotations is the spider gears absorb ½ of the rotating movement.

Check Gear Ratio with a LSD.
Jack up BOTH tires and secure with jack stands.
1. With the driver’s side tire still in the air (and jack stands in place), rotate the tire until the valve stem is at 12:00.
2. Roll under the truck and chalk mark the yoke and differential case at the 3:00 or any where you will be able to see it from the tire.
3. Now roll back to the tire and while watching the chalk mark, rotate the tire exactly 1 full rotations. Use the valve stem that was at 12:00. Count the full rotations of the drive shaft.
4. Now, the valve stem is back at 12:00, but the chalk mark is not at 3:00. Roll under the truck and determine the partial turn. It could a half turn, it could be a three-quarter turn, or maybe a tenth of a turn.
5. Add the full turns to the fraction of a turn and you have you ratio. 3-3/4 is 3.75:1.
6. There are only a few actual gear ratios and this will get you within a few hundredths.

Hope this helps.

[WorkinLonghorn]

Thanks for posting this. It seems to come up pretty often so maybe it should be a FAQ (or maybe it is already).

Why does everyone go straight for the messiest way to determine gear ratio?

I used to ask that too but it does make sense to encourage a guy to change the lub if it's a newly acquired truck with unknown quality gear oil in there.

[LockDoc]

-
Here is another way:

There is a code stamped on the passengers side axle tube. Pic included. If you find a two or three letter code go to this web site to look it up. You only need to use the first letters, some have two, some three. depending on the year..

http://www.americanclassic.com/paint_test.asp

LockDoc