put mark on tire and one on the drive shaft at the bottom.I count the revolution of drive shaft with one turn of the wheel..only getting a little under 2 1/2 turns? does this mean i have a 2:40 ratio posi? ive never seen one with this kind of a ratio? car runs strong with this rear end but wonder what 3:40 or 3:73 would run like. have a 402 with 292h comp cam 800 holley and 2800 stall. anyone seen or run a posi with this kind of gearing?:D

Both rear wheels have to turn exactly one revolution or if one wheel is stationary the other has to turn exactly 2 revolutions.

If you have a positrac, both rear wheels will turn together.
If this is the case, mark the drive shaft and and the inside of one wheel and backing plate.

Turn the drive shaft until the wheel makes one turn. If the drive shaft makes 3 1/2 turns for one turn of the tire you have a 3.55, and so on.

If you do not have a positrac block one tire so it does not turn.
Mark parts as above.

Turn the drive shaft until tire makes one revolution.
If the drive shaft turn 1 3/4 turns, multiply by two, equals 3 1/2, you have a 3.55 rearend, etc......

If you do not have a positrac block one tire so it does not turn.
Mark parts as above.

Turn the drive shaft until tire makes one revolution.
If the drive shaft turn 1 3/4 turns, multiply by two, equals 3 1/2, you have a 3.55 rearend, etc......

Pretty sure this bit is backwards -- you need to divide by 2, not multiply. <Ignore this part, it's incorrect>
Axle ratio = input speed / (average rear wheel speed).

On a clutch-type posi, both wheels turn at the same speed, so average is the same as a single wheel.

On an open diff with one wheel blocked, average is half the number of turns of the non-blocked wheel. So it takes half as many turns of the drive shaft for a single turn of the wheel. Axle ratio is then drive shaft turns times 2


Having said all that, if you really did have both wheels turning, then you may have a 2.56 posi. GM did offer these in various vehicles.

It's two turns of the wheels, the sum of both wheels. Hold one still and turn the other one two turns, or turn them one turn each and count the number of turns of the driveshaft. If you want more accuracy, do twenty turns.

If you want to be absolutely sure, drain the rearend pull the cover off the differential and count the teeth on the ring gear and divide that number by the number of teeth on the pinion. Gives you the ratio.

Of course you'll have to replace the fluid, which normally isn't a bad thing.

or...you could take the numbers off the axle tube and decode it

If you have a positrac, both rear wheels will turn together.
If this is the case, mark the drive shaft and and the inside of one wheel and backing plate.

Turn the drive shaft until the wheel makes one turn. If the drive shaft makes 3 1/2 turns for one turn of the tire you have a 3.55, and so on.

If you do not have a positrac block one tire so it does not turn.
Mark parts as above.

Turn the drive shaft until tire makes one revolution.
If the drive shaft turn 1 3/4 turns, multiply by two, equals 3 1/2, you have a 3.55 rearend, etc......

Verle has it correct...............

If you have a positrac, both rear wheels will turn together.
If this is the case, mark the drive shaft and and the inside of one wheel and backing plate.

Turn the drive shaft until the wheel makes one turn. If the drive shaft makes 3 1/2 turns for one turn of the tire you have a 3.55, and so on.

If you do not have a positrac block one tire so it does not turn.
Mark parts as above.

Turn the drive shaft until tire makes one revolution.
If the drive shaft turn 1 3/4 turns, multiply by two, equals 3 1/2, you have a 3.55 rearend, etc......

Verle has it correct...............

Why not turn the driveshaft until the open rear axle wheel makes two revolutions and measure it directly? The more turns, the more accurate it your measurement will be.

Why not turn the driveshaft until the open rear axle wheel makes two revolutions and measure it directly? The more turns, the more accurate it your measurement will be.

I agree with you........two wheel turns is the same as multiplying by 2 and should be more accurate.

Verle has it correct...............

Crap, I'm an idiot. Now that I'm semi-conscious, I retract my previous statement. I'm used to turning the wheel 2 times (not the driveshaft) so I got all my math backwards. Stupid algebra.

Turning the drive shaft two full turns will give the same result as one turn and multiplying by two.

Turning the drive shaft two full turns will give the same result as one turn and multiplying by two.

I'll have to try the above next time I have an opportunity but I'm skeptical it will work........are you sure you mean drive shaft two turns and multiply wheel revolutions?

if you Really want to know ........

remove the rear cover & count the Ring & Pinion teeth :yes:

the gear lube probbly hasn't been changed in 40 years anyway ;)

Turning the drive shaft two full turns will give the same result as one turn and multiplying by two.


This isn't correct

Your method with a 3.42:1 gear ratio if you turn the drive shaft one revolution the both wheels would turn .29 revolutions. Two turns of the drive shaft would rotate the wheels .58 revolutions. No useful information.

Correct
Turn the carrier '1' revolution. The amount that the pinion/drive shaft rotates is the gear ratio. ie. With one revolutions of of both wheels the pinion turns 3 revolutions the ratio 3:1, 4 revolutions 4:1, 3.5 revolutions 3.5:1 etc.

If both wheels are turning at the same rate the carrier is turning at that rate also. With one wheel stationery the carrier turns at 1/2 speed of the turned wheel. The turned wheel will have to rotate '2' revolutions for the carrier to turn '1' revolution.

if you Really want to know ........

remove the rear cover & count the Ring & Pinion teeth :yes:

the gear lube probbly hasn't been changed in 40 years anyway ;)

Man, I'd just pop the cover take a look. Then you'll know for sure. I agree, the fluid probably hasn't been changed in years anyway.

I realize this post is starting to get confusing, so I'll throw some more regurgitation at you.
Main confusion for new guys attempting to calculate their ratio is not understading that it makes a difference if you are running a posi/locker/spool compared to an open diff.
The math for the posi/locker/spool is straight forward becase the wheels will turn together. If the driveshaft turns a touch over 4 times for the wheels to make one rotation, you likely have a 4.11.

Now what is important if you do have an open (you will know if you have an open because if you turn one wheel forward, the other will go backwards)........... yes both wheels are turning, but they need to be going the same direction to have a posi/locker/spool. If they are going in two different directions you have an open rear. In this case block one wheel or have someone hold other other. By doing this, you can not use the same math as you did on the posi/locker/spool because of the spider gear set up in the rear of an open. It messes with your numbers. Then you will need to use the conversation as discussed earlier. This is because of how the spider gear is working. Now one input of the driveshaft is turning the wheel twice as fast as it would if they other tire were free to rotate. Thus the number of your driveshaft inputs to wheel rotations will need to be halved. This is also why you can do awsome "one wheel wonder" burnouts in an open rear. That rear tire is spinning rather fast.

There are two ways of doing these numbers........turning the wheel and watching the shaft or turning the shaft and watching the wheel. I personally like to turn the shaft because the numbers make more sense to me because I will get the 3.42 like my car has. I turn the drive shaft 3.42 times for the wheels to make their full one turn (I have a locker). This makes sense to me because that is what a rear ratio is telling you. A rear gear ratio is the ratio of (input revs)/(output revs). Thus input rev of driveshaft (3.42) / output rev of axle (1). 3.42/1= 3.42
This is why some guys will spin the wheels more, it will give you a more accurate read.
For example, if I were crazy I could desire to count the number of driveshaft turns to turn to get 100 tire rotations. I could do this and I'd cound 342 driveshaft turns. 342/100=3.42 again.