Adding Posi '68 C20 - The 1947 - Present Chevrolet & GMC Truck Message Board Network

Astro-Balls · 04-15-2020, 06:09 PM

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?

Bigdav160 · 04-15-2020, 06:29 PM

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

Astro-Balls · 04-15-2020, 06:33 PM

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?

**jocko** · 04-15-2020, 07:18 PM

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.

Barnfind46 · 04-15-2020, 07:53 PM

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.

**Corts60** · 04-15-2020, 08:01 PM

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

**Boog** · 04-15-2020, 08:16 PM

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.

Astro-Balls · 04-15-2020, 08:41 PM

Tire size is P265-75 R 16

**jocko** · 04-15-2020, 08:56 PM

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.

weim55 · 04-15-2020, 11:28 PM

Quote:

Originally Posted by Astro-Balls

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?

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

Astro-Balls · 04-16-2020, 12:27 AM

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.

PepperTreeGarage · 04-16-2020, 12:47 AM

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

Astro-Balls · 04-16-2020, 01:18 AM

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

**special-K** · 04-16-2020, 07:22 AM

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

PepperTreeGarage · 04-16-2020, 06:59 PM

Quote:

Originally Posted by Astro-Balls

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

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

Astro-Balls · 04-17-2020, 10:02 AM

Quote:

Originally Posted by special-K

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

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.

Astro-Balls · 04-18-2020, 03:55 PM

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

**jocko** · 04-18-2020, 11:14 PM

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)

Astro-Balls · 04-18-2020, 11:19 PM

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

RichardJ · 04-19-2020, 07:50 PM

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.

04-15-2020, 06:09 PM	#1
Astro-Balls Registered User Join Date: Feb 2019 Location: Nor Cal Posts: 822	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? __________________ 1968 C20 Fleetside 396 4sp PS PB AC CST . https://www.youtube.com/watch?v=RWK8GgWD4uA

04-15-2020, 06:29 PM	#2
Bigdav160 Registered User Join Date: Apr 2011 Location: Klein Texas Posts: 3,852	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 __________________ My Classics: '72 K20 Suburban + '65 Dodge Town Wagon '72 Corvette Roadster +'67 Corvette Roadster '73 Z-28 Camaro '63 Ford SWB Uni Pickup '50 Ford Coupe

04-15-2020, 06:33 PM	#3
Astro-Balls Registered User Join Date: Feb 2019 Location: Nor Cal Posts: 822	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? __________________ 1968 C20 Fleetside 396 4sp PS PB AC CST . https://www.youtube.com/watch?v=RWK8GgWD4uA

04-15-2020, 07:18 PM	#4
jocko Senior Member Join Date: Oct 2009 Location: Godley, TX Posts: 17,945	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. Last edited by jocko; 04-15-2020 at 07:27 PM.

04-15-2020, 07:53 PM	#5
Barnfind46 Registered User Join Date: Aug 2014 Location: Mena,Arkansas Posts: 485	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. __________________ 68' with no bells or whistles unless I add'em.

04-15-2020, 08:01 PM	#6
Corts60 Just here to tinker Join Date: Jun 2008 Location: Holland, MI Posts: 3,689	Re: Adding Posi '68 C20 I have 3:73 with no overdrive and 30" tires. My engine RPM's are about 2800 at 70. __________________ 78 C10 SWB Diesel #1 (wrecked)http://67-72chevytrucks.com/vboard/s...1978+silverado 78 C10 SWB Diesel #2 (sold)http://67-72chevytrucks.com/vboard/s...d.php?t=650751 60 GMC Suburban (sold)http://67-72chevytrucks.com/vboard/s...d.php?t=445526 60 GMC 1000 (sold)http://67-72chevytrucks.com/vboard/s...d.php?t=298235 67 GMC 1500 (sold) http://67-72chevytrucks.com/vboard/s...d.php?t=803695 86 Suburban(summer unit) http://67-72chevytrucks.com/vboard/s...d.php?t=810697 88 Suburban 4x4(winter unit)https://67-72chevytrucks.com/vboard/...25#post8900725

04-15-2020, 08:16 PM	#7
Boog laying low Join Date: Mar 2001 Location: Searcy, Ark. USA Posts: 13,092	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. __________________ Boog 69 Chevy stepside, 358/T350, 4.11 posi, 4.5/4 drop, rallys, poboy driver primer is finer 91 Chevy sportside, Tahoe, Yukon & GMC Crewcab All GM..'nuff said. I stand for the flag and kneel at the cross

04-15-2020, 08:41 PM	#8
Astro-Balls Registered User Join Date: Feb 2019 Location: Nor Cal Posts: 822	Re: Adding Posi '68 C20 Tire size is P265-75 R 16 __________________ 1968 C20 Fleetside 396 4sp PS PB AC CST . https://www.youtube.com/watch?v=RWK8GgWD4uA

04-15-2020, 08:56 PM	#9
jocko Senior Member Join Date: Oct 2009 Location: Godley, TX Posts: 17,945	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.

04-16-2020, 12:27 AM	#11
Astro-Balls Registered User Join Date: Feb 2019 Location: Nor Cal Posts: 822	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. __________________ 1968 C20 Fleetside 396 4sp PS PB AC CST . https://www.youtube.com/watch?v=RWK8GgWD4uA

04-16-2020, 12:47 AM	#12
PepperTreeGarage Registered User Join Date: Jan 2014 Location: SoCal Posts: 281	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 Attached Images

04-16-2020, 01:18 AM	#13
Astro-Balls Registered User Join Date: Feb 2019 Location: Nor Cal Posts: 822	Re: Adding Posi '68 C20 Nice job, What gearing did you go with? Engine and Trans? __________________ 1968 C20 Fleetside 396 4sp PS PB AC CST . https://www.youtube.com/watch?v=RWK8GgWD4uA

04-16-2020, 07:22 AM	#14
special-K Special Order Join Date: Nov 2004 Location: Mt Airy, MD Posts: 85,862	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 __________________ "BUILDING A BETTER WAY TO SERVE THE USA"......67/72......"The New Breed" GMC '67 C1500 Wideside Super Custom SWB: 327/M22/3.42 posi.........."The '67" (project) GMC '72 K2500 Wideside Sierra Custom Camper: 350/TH350/4.10 Power-Lok..."The '72" (rolling) Tim "Don't call me a redneck. I'm a rough cut country gentleman" R.I.P. ~ East Side Low Life ~ El Jay ~ 72BLUZ ~ Fasteddie69 ~ Ron586 ~ 67ChevyRedneck ~ Grumpy Old Man ~

04-18-2020, 03:55 PM	#17
Astro-Balls Registered User Join Date: Feb 2019 Location: Nor Cal Posts: 822	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 __________________ 1968 C20 Fleetside 396 4sp PS PB AC CST . https://www.youtube.com/watch?v=RWK8GgWD4uA

04-18-2020, 11:14 PM	#18
jocko Senior Member Join Date: Oct 2009 Location: Godley, TX Posts: 17,945	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)



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04-18-2020, 11:19 PM	#19
Astro-Balls Registered User Join Date: Feb 2019 Location: Nor Cal Posts: 822	Re: Adding Posi '68 C20 Thanks, I think everyone confirmed its geared a little low.. Now to get it changed out.. __________________ 1968 C20 Fleetside 396 4sp PS PB AC CST . https://www.youtube.com/watch?v=RWK8GgWD4uA

04-19-2020, 07:50 PM	#20
RichardJ Registered User Join Date: Apr 2010 Location: So Cal Posts: 1,420	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. __________________ '67 GMC 2500, 292, 4spd, AC

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