8 3/4" gear ratio question

[73 ralley]

My question is; is there a way to tell what gear ratio my 8 3/4 has? The casting numbers say 2881489  AAPW There is no tag on it showing ratio. Is there another way aside from opening it up. They are very low as 60 MPH is about 2200 rpm. Open to your suggestions.

[Kelly B.A.]

Can jack up rear of car(both wheels with sure grip)and do the old mark the drive shaft,and mark the tire,turn drive shaft and count until wheel comes around.
From what you said I would say 3.23 or lower(numeric-lower)

[383barracuda]

It looks like my Plymouth Fury.
60 MPH and 2200 rpm.

I have a 383 with a 727.
8 3/4 rear axle with 489 casing with 2.76 ratio.

[Chryco Psycho]

if you have a non sure grip diff or both tires do not turn equaly you need to block 1 wheel & count the revolutions of the driveshaft to make 2 full turns of one tire to be accurate
 the possible ratios are 3.23 , 2.91, 2.76

[shadango]

I have  318 with the 8.75 rear and a 727....my RPMs are around that 2200-2300 rpm mark at 60 mph or so and I have 2.76s.

[73 ralley]

Just so I understand this, if it is a sure grip then both tires will spin the same direction and the same revolutions and if not a sure grip then the other tire will spin the opposite direction or just not the same amount of revolutions.
Thank you all for your help on this.

[MEK-Dangerfield]

Just so I understand this, if it is a sure grip then both tires will spin the same direction and the same revolutions and if not a sure grip then the other tire will spin the opposite direction or just not the same amount of revolutions.
Thank you all for your help on this.

You have it right. I would tend to agree with you having 2:76 gears also. My 3:23's put me at 2500-2600RPM at 60 MPH.

[Moparal]

Im doing 48 mph at 2200 and 60 is 3000 with 275 60 15's  27.9 tall  with 410 gears.  I usually run around 3500 on the street tho

[73 ralley]

Update, I jacked her up and both tires spin the same direction and I get just over 2 1/2 revolutions of drive shaft to 1 rotation of tire. 2:73's i'm guessing. Next question is, what degree of difficulty would it be for me to pull the pot out and have 3:7's or 3:9's put in and I reinstall it? Is that advisable for someone to do that has to ask if its advisable to do? Is it that easy or is it best left for the pros?   

[Chryco Psycho]

if you are going to swap a carrier completely set up into the diff no problem to do , you just need to set the end play @ .008 -.012
 if you want to have gears installed into the carrier you have I would have a pro do them for you

[Moparinho]

I'm just finished to do the same test because I always guessed to have 3.23:1 without any real check.

So, I jacked the car with one rear wheel still blocked on the gorund (I should not have SureGrip..).
The result was 1,5 revolutions of the driveshaft  to complete 1 revolution of the wheel. 

At this point I'm going to be little bit confusing... what kind of gear ratio should I have ?  there is something wrong in my testing ?

[73 ralley]

I will leave the answer to your questions to the pro's onboard but I did my test with both wheels off the ground not just one. I was under the understanding when you do this and both wheels turn the same direction its posi and if they go in opposite directions its not posi. This is only what i'm told by friends with the same experience as me. Maybe some pro's could confirm this.

[Moparinho]

I'm just finished to do the same test because I always guessed to have 3.23:1 without any real check.

So, I jacked the car with one rear wheel still blocked on the gorund (I should not have SureGrip..).
The result was 1,5 revolutions of the driveshaft  to complete 1 revolution of the wheel. 

At this point I'm going to be little bit confusing... what kind of gear ratio should I have ?  there is something wrong in my testing ?

maybe because I turned the wheel to check the driveshaft instead of turning the driveshaft directlly ??

[Tonefiend]

You are going to have to have a pro change the gears and set up the unit for you.  Figure $150 for the gears and $150-$200 for the labor.  While in there the mechanic will check your other bearings, seals and posi unit and may or may not find other stuff that should be replaced while you are in there.  So figure $300 minimum, possibly more.  Your sure grip may need a rebuild, or if it's a clutch it may be on the last leg, especially if you are putting out some hp.  Your yoke may be worn, so the possibility is there you will need a sleeve or seals.  Also I don't know if they need to replace the pinion crush sleeve and seals when you change gears, but there is some extra $$$

 Replacing it is not a big deal. You will need to replace the inner seals while you are in there which is slightly tricky.  You will need a seal press, or improvise one.   Also it is very heavy so you will need to accomodate that with a jack, or extra set of hands. 

I just went through replacing mine and it was doable and I am not much of a mechanic.  One other issue will be removing the brakes. I had discs in the back, so I had to take them off to get the axles out.  My mechanic needed my axles so he could test my sure grip.