Adding Posi '68 C20
 

Truck now has open rear, want to add Posi and different rear gear.
calculating current rear gear, one turn of the tire jacked up = 2 1/4 turn of the shaft.. 2.73 gears??
I'm thinking going to 3.55 rear gear,
Go with Eaton Posi?

Re: Adding Posi '68 C20
 

Pictures of your axle? If you have a dana you are in good shape? If it's a factory big block truck it just might. And a 3.54:1 was available. Not a 2.73:1

edit, the power team chart says it should be already be a 3.54:1. That was all that was available with a C20 396 powered truck

Re: Adding Posi '68 C20
 

Yes, Its a Dana 60. 4 speed NP435.
The trucks top speed wants to be about 55mph, anything above that it starts taching up pretty high so I'm sure its not good for fuel mileage.
Would the original rear gears be on the SPID? build sheet?

Re: Adding Posi '68 C20
 

Won't be on your SPID. Gear ratio for a 68 big block was 3.54 in a Dana 60 (there were no optional ratios for a 396 C20 in 68 as Bigdav mentioned)
Just a note - HB8 NoSPIN was not an option for 396, only G87 POWR-LOK.
Highly unlikely anyone would (or could) swap in 2.73 ratio.
What are your highway rpms at a 55, if you know? The fact that you're topping out at 55 pretty much eliminates the 2.73 theory. Possible someone swapped in an aftermarket 4.10/4.57 r&p etc. 3.54 gears wouldn't have you at an uncomfortably high rpm at 55, depending on tire size.

Re: Adding Posi '68 C20
 

I am running a late model 14 bolt but my 3.73 gears with 30 in tires is not bad at 70. Not running an overdrive so third gear is 1 to 1. Would probably like 3.5-somethings better though.

Re: Adding Posi '68 C20
 

I have 3:73 with no overdrive and 30" tires. My engine RPM's are about 2800 at 70.

Re: Adding Posi '68 C20
 

Without knowing just what the rpms are at 55 it may just be that it sounds like it's really spinning up there as compared to today's vehicles with overdrive? With tire size and road speed one of you guys could calculate the rpms with the stock gear ratio for him.

Re: Adding Posi '68 C20
 

Tire size is P265-75 R 16

Re: Adding Posi '68 C20
 

rpm for that tire size (~31.5 dia) at various speeds with a 3.54 would be:
2077 @ 55 mph
2454 @ 65 mph
2832 @ 75 mph

If you happen to know your rpm at any given speed, can tell you what your rear ratio actually is.

Re: Adding Posi '68 C20
 

When you checked the driveshaft turns to one turn of the wheel: one side jacked up and the other on the ground you double the drive shaft turns to get the actual ratio. 2.25 x 2 = 4.50

4.56 ratio, maybe somebody swapped those in at some point?

Steve weim55 Colorado

Re: Adding Posi '68 C20
 

Yes, One tire, the right rear was on the ground and I turned the left.
I will check the tach to see what it reads at 55 and 60-65. But the revs seem quite high above 60-65.

Re: Adding Posi '68 C20
 

As been said, lots available for the DANA 60. Not cheap though. We Rebuilt everything. Used Yukon gears and carrier. Torque King was a good source for us. https://torqueking.com/?cmp_id=17657...SAAEgJwhfD_BwE

Re: Adding Posi '68 C20
 

Nice job, What gearing did you go with? Engine and Trans?

Re: Adding Posi '68 C20
 

It sounds like you don't have 3.54s. It must not be the original rear. 3.54s are the best you'll find, so look for that rear or ring & pinion. If you learn you still have original rear, then your already at your best ratio

There is a great variety of posi-tractions available for the Dana60

Re: Adding Posi '68 C20
 

3.73--4L80E--Haven't fully decided on engine. Most likely a 383...……….

Re: Adding Posi '68 C20
 

Thanks very much, I think 3.54's are the way to go. will call the local rear end shop to see about installation.
there is a shop near me (rear end specialties) in San Jose
That does good work.

Re: Adding Posi '68 C20
 

Checked the Tach readings..
1st- Granny gear.
2nd- 2700 at 25 mph
3rd- 2400 at 40mph
4th-2500 at 55 mph
4th -2700 at 60mph
4th -3300 at 70mph

Re: Adding Posi '68 C20
 

a 4.56 would be ~3400 at 70 mph in your scenario, so that's what I'm gonna guess it has. (4.10 would be just under 3100 rpm at 70 mph)

Re: Adding Posi '68 C20
 

Thanks, I think everyone confirmed its geared a little low..
Now to get it changed out..

Re: Adding Posi '68 C20
 

My '67 3/4 T, 292, 3 spd, Dana had 4.56 gears. I found a late '70s Dodge 3/4 in the wrecking yard with a Dodge Dana with 3.54 gears and posi carrier.

The gears and posi carrier was $50.
New bearings, seals, gaskets and shims was $120.

Axle splines turned out to be the same (lucky). Dodge pinion splines are different and Dodge used smaller U-joints than GM. A rear end shop had a new, bigger yoke for $20.

Prices are from 2005.

My 6 cyl truck runs easily at 75 mph with the 3.54 gears and 285/70R17 tires.