need help with axle ratio

[292farmer]

i have a 1972 gmc c20 292i6 4 spd manual reg cab lwd i need help trying to figure out what my gear ratio is, i am told that it was a 4.56 by original owner i found these numbers and letters, here they are sorry some pics are upside down.
c34950 2 60-3b it has ten bolts im pretty sure its a dana 60 hence the the 60 num
i used a formula to figure out my ratio using my rpms and mph and i came up with 5.3? i thought maybe 5.13? but didnt see it as option for dana 60

[msgross]

I always suggest to taking the diff cover off, look at the numbers on the ring gear (41:11 ratio = 3.73, etc..) and put some fresh oil when you install the cover again...

[292farmer]

yeah that was the next step i was just curious to see if what i could figure out off the numbers did the dana have posis? cuz it has in sane traction and neither wheel will spin much before they both seem to match each other

[69GMCLonghorn]

I'm guessing the one thing that might be skewing your data is that most speedo's and tach's in these trucks are not 100% accurate (unless you're running an aftermarket tach). In mine, the speedo was off by around 12%. the tach was off by 20%!!!! So find a way to calibrate yoru gauges and try it again.

[msgross]

gmc's did have dana60's so you're probably right with that one, would need more pics though...

Once you open it up and snap some pics you will see what it has in it...

[292farmer]

if i remember ill take more pics tomm, if im not to tired and can afford the gasket and oil after work maybe ill peel off cover too. as for the tach it was a sunpro my dad lent me for the test (out of his 250i6 powered buggy) and the mph were done by radar by the local cop i know him and he was happy to help my speedo was only 2 mph off. so thats not bad, truck appears to have only 87000mi unless its turned over already.

[leddzepp]

Easy to figure out both posi, and gear ratio:


Jack the truck's back end up. Spin the tires. If they both spin in the same direction, you have posi. If they spin opposite of each other, you do not have posi.

Figuring out your gear ratio is easy. Attach one lug nut on one of the wheel studs for reference. I put it in the 12 o'clock position. You can mark a spot on the driveshaft for reference, or use the pinion yoke for reference if you do not have the driveshaft attached. If your driveshaft is attached, your trans will need to be in neutral. Turn your driveshaft and count the number of times it takes for that lug nut to reach the 12 o'clock position again. Mine took 3 3/4 turns, so I have a 3.73
If it takes 3 turns, you have a 3.07, 3 1/2 turns you have a 3.43 (unlikely), 4 turns, you have a 4.10, and so on.....

[capev86]

are you sure it is a dana 60 and not the corp/eaton HO52.....i think the dana came only on the trucks with big block or special ordered with the 3.54 axle ratio. the corp axle has a perfectly round pumpkin with 10 bolts on the diff cover and it came with a 4.10 or 4.57 (not 4.11 or 4.56) gear on a 3/4 ton truck.

[292farmer]

heres some pics pretty sure a dana not an eaton, so not a posi but...tried that jack it up trick and first time 2 turns second and third to double check still only barely two turns?? spun wheel clockwise and counter then tried again still only two turns? definetly not right since it screams above 45mph

[leddzepp]

Don't spin the wheel.....turn the driveshaft until the wheel makes one full revolution. Count how many times the driveshaft turned to get the wheel to rotate one full turn.

[Ricks' 72C10]

Originally Posted by leddzepp

Don't spin the wheel.....turn the driveshaft until the wheel makes one full revolution. Count how many times the driveshaft turned to get the wheel to rotate one full turn.

Ledzepp

I'm trying to figure out my gear ratio also

I marked the tire at 12:00 and turned the diff yoke, it took only 2.3 turns to come back to the top, less than 2 and a half. I tried it 3 times cause I couldn't believe it. What ratio do you think this one is?... A 2.30? It is factory stock '72 C10, it had straight 6 and 3 on the tree.

Thanks

[292farmer]

thanks ledd zepp ill try that tomm. do u know how i can tell whether i have the ch/sm465 4spd or the np435 4spd? im told the np was more common in the gmc's the glovebox code says M20 4 speed trans. its not a huge deal with the trannismission im more curious then anything thanks!

[1968CHEVYC10]

Originally Posted by Ricks' 72C10

Ledzepp

I'm trying to figure out my gear ratio also

I marked the tire at 12:00 and turned the diff yoke, it took only 2.3 turns to come back to the top, less than 2 and a half. I tried it 3 times cause I couldn't believe it. What ratio do you think this one is?... A 2.30? It is factory stock '72 C10, it had straight 6 and 3 on the tree.

Thanks

if you have an open diff, and only wheel is spinning while your doing this, then you need to double it.

[Ricks' 72C10]

Originally Posted by 1968CHEVYC10

if you have an open diff, and only wheel is spinning while your doing this, then you need to double it.

Yep...

Only the drivers side wheel was turning, so you mean maybe it is more like a
4.60 more or less?

Thanks

[1968CHEVYC10]

Originally Posted by Ricks' 72C10

Yep...

Only the drivers side wheel was turning, so you mean maybe it is more like a
4.60 more or less?

Thanks

yea probably a 4.56