Please check my math: Rear End Gearing

[Nuckingfuts]

So we're still getting a pulse on this truck's (73' shortbox) specs & general health and so this weekend we are going through a laundry list of checkpoints, one being to I.D. the rear end. The only markings I could find so far on the rear end are 3896839 and 'N' just below it, these markings were on the forward/passenger side of the pumpkin. After some research I came up with no clear answer and therefore tried the 'jack it and count it' method.

With both rear tires off the ground I could spin one tire forward and watch the other spin in reverse......but only for about one rev before the other tire stopped spinning. The opposing wheel spin makes me think it's an open diff but the fact that the opposing tire stops spinning in reverse after some initial rotation has me confused.

Let's set that aside, counting the wheel rev's to drive line rev's I came up with this:

Method 1:
Both tires off the ground and just turning one tire while the other was allowed to free turn (actually it only turned for a partial rev then no longer rotated during test).
8 wheel rev's : 12.25 rev's

Method 2:
Both tires off the ground but one wheel being held static while the other is rotated.
8 wheel rev's : 12.25 rev's

Both ratios appear to equal 3.08s....am I wrong here?

I'm not ready to open the rear end just yet to verify so I thought I would run it by you guys first. Is this open diff? Do I have a good estimate to assume 3.08 gearing? Thoughts?

[cadillac_al]

Your math sounds right but what do you have against changing the gear oil? I usually change all fluids when I buy a vehicle just for peace of mind.

[mongocanfly]

Its to early for me to be doing math in my head but by jacking up one wheel and leaving the other on the ground...just rotate the wheel one revolution while counting the driveshaft rotation...if youve got 3.08s you should have just over 3 full driveshaft rotations for one rotation of the wheel...but if you want to be 100% sure pull the cover..gaskets are cheap enough..probably needs rear dope refreshed anyway

[75C10]

NF,
Method 2 works fine with non clutch posi (i.e. open) rear ends. One wheel held static. Spin the other tire two tire rotations and count drive shaft rotations. You have 3.08. The two turns come as a result of the spider gears. One tire rotation would be true if there was a clutch type posi installed (and both tires rotated in the same direction).

[70K10Hunter]

Originally Posted by 75C10

NF,
Method 2 works fine with non clutch posi (i.e. open) rear ends. One wheel held static. Spin the other tire two tire rotations and count drive shaft rotations. You have 3.08. The two turns come as a result of the spider gears. One tire rotation would be true if there was a clutch type posi installed (and both tires rotated in the same direction).

What?

[andyh1956]

He means the spider gears have half as many teeth as the side gears. By only turning one tire you have effectively created a planetary gear set. With only one tire being rotated the axle is driving the side gear with more teeth than the spider & it is rotating the carrier by the cross shaft AROUND the stationary axle, which can drive the PINION FASTER than it would if all the carrier gears were stationary like when traveling down the road in a straight line. It is Best to turn the drive shaft & make sure BOTH tires rotate together. This can involve multiple (Count Three) peoples working together but it is the most accurate. Just put the truck in neutral on level hard ground, mark the rear tires at 6 o'clock & PUSH the truck until the tires make one revolution while some one watches the pinion rotate & there you go!
If the tires make one revolution & the pinion turns four times = 4.00 x 1 ratio!

[75C10]

andyh thank you

Just jack up one wheel with the other on the ground. Place valve stem at the bottom and use this as your turning reference point. Now you don't need to look at the wheel to know it position. Turn wheel twice and count drive shaft revolutions. Easily done by yourself in most instances. Round as best possible and voila. Easy Peasy and right every time.

[wilkin250r]

The opposing tire might just have a little sticky spot in the brake drum, a little corrosion maybe. I wouldn't worry about it just yet, I think you have bigger issues.

And your math sounds correct. If one tire is held static while the other is turned, then due to the actions of the spider gears, 8 rotations of the tire will result in 4 rotations of the ring gear. 12.25/4=3.0625

My extensive research (43 seconds) also says the 12-bolt came in a 3.07 variety (43:14), but I think most aftermarket gear sets are 3.08 (40:13).