Rearend gear dont know what I have

A question for you all I'm trying to figure out what gear is in my 9". The one thing I tried and was told to try was jack the rearend up rotate one tire a full revolution and at the same time count the pinion to see how many times it goes around I did and what I was coming up with is 2 full rotations. So what does that mean? I've checked another rear end I have laying around and 1 full rotation on the wheel equaled 3 3/4 turnes on that rear end and I know it has a 3.73 gear so that rear end is checking fine. So what about my first rear end it came out of a 63 ford unibody truck with a 6 cyl 3on the tree 2 turns on the pinion does'nt make sense to me any help would be great.THANKS

 

Open differentials lock one wheel and count 2 revolutions from the other. The spider gears act as a 2:1 gear.
Think of it as when they spin together it is 1:1 if only one tire spins that one has to spin 2x faster to make up for the other one not spinning.

Does that make sense?


If you have a posi or spool and both tires are spinning then you can jack both tires off the ground and you just count 1 tire revolution.

You get close like that like you said 3 3/4 is about 3.73
If you want to know for sure count the ring gear teeth and divide by pinion teeth.

Hope this helps.
~Justin

 

Because of the differential action, you need to multiply the results by 2. Sounds like a 4.11

 

http://www.csgnetwork.com/rnggrpinioncalc.html

 

Here you go...

http://www.how-to-build-hotrods.com/gear-ratio.html

 

3.89 or 4.10 ratio, is my guess.

If you want to get it exact, turn that one tire 20 times, and count the pinion revolutions exactly. then divide by 10.

 

Ok so for open differentials rotate the tire twice and count the pinion at the same time that makes since because the second rear end I tried this on is does have a posie and it was coming out right. Thank you so much I was getting pretty confused.