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10/19/2020 7:16 PM  #1


I bought a 8" with a auburn posi unit in it. I pulled the center section out and I wanted to verify what I was told the gear ratio was 3.80 the numbers on the ring gear are 82-0710-0, also was a, K 7 8620. I can't seem to find anything on these numbers.


10/19/2020 7:56 PM  #2

Re: richmond

You can count the teeth and divide the ring gear by the pinion gear to get a gear ratio.

John  -- 67 Mustang Coupe 390 5 speed

10/20/2020 5:05 AM  #3

Re: richmond

I get that but I was wanting to identify the numbers on the ring gear, and since it is all installed I figure i'd miscount  somewhere along the way.

     Thread Starter

10/20/2020 8:29 PM  #4

Re: richmond

I’ve counted wrong more than once!!! I now start with a reference mark by pencil or sharpie and count both at least twice or until I get something repeatable.

John  -- 67 Mustang Coupe 390 5 speed

10/24/2020 5:17 PM  #5

Re: richmond

Also, it is easy enough to pull the pinion assembly so you can count the pinion teeth. Does not change set-up.  Probably needs a new o-ring in there anyway.

Money you enjoy wasting is NOT wasted money... unless your wife finds out.

10/25/2020 5:33 AM  #6

Re: richmond

If it's a non-hunting gear ratio then the pinion must be placed back into the same spot on the ring gear in which it was removed.


10/25/2020 2:28 PM  #7

Re: richmond

Time to count!  Those numbers you posted thwarted my google skills.


10/25/2020 6:07 PM  #8

Re: richmond

I have tested differential ratios with the drive train in place.  It is a best guess, but has worked for me.

Have the rear tires in the air first and blocks in front of the front tires and anything else you want for good safety.

Put the trans in neutral.

put a straight mark with crayon or something across the snout of the 3rd member and the drive shaft u-joint.  Same with a tire and some close object like a backing plate.

Start turning the drive shaft slowly, counting the number of turns needed to turn the tire one rotation.

Other than a 3.00 rear end you will have to estimate the degrees the u-joint turns,  Things like half turn- 3.50, quarter turn 3.25.  You will get something close to what the ratio is. 

It is not good enough to tell the diff between a 2.79 & 2.80, but you can tell if you have a 3.40 or a 3.50. Just make sure both tires are turning at the same rate.  If not all bets are off.  It helps to know what you think you might have as Ford has a fixed set of ratios that you can use for reference.

Original owner - 351w,T-5, 4whl disks, power R&P

10/25/2020 6:59 PM  #9

Re: richmond

He's already pulled the center section out so, counting ring and pinion gear teeth are easy...

.......common sense is a flower that doesn’t grow in everyone’s garden.......

10/26/2020 6:12 AM  #10

Re: richmond

i used a solvent on the op/inion and marked it with a sharpie didn't think I could actually get a sharpe in there, but pinion 10 ring gear 38. just like the guy told me 3.80

     Thread Starter

10/26/2020 9:53 AM  #11

Re: richmond

lowercasesteve wrote:

Just make sure both tires are turning at the same rate.

Years ago I was told that with a non-posi rear if you hold one tire from turning, the ratio you get will be 1/2 the actual ratio.  I tired it and it worked out.

65 Fastback, 351W, 5-speed, 4 wheel discs, 9" rear,  R&C Front End.

10/26/2020 12:23 PM  #12

Re: richmond

bob e interesting I was wondering how to tell that on a open rear end.

     Thread Starter

Board footera

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