POSI Question

[LT1 Burb]

Will a 30 spline carrier and gear set from a 66 chevy truk 12 bolt fit in my 72 12 bolt in my suburban. I want to re-gear the rear from 3:08 to 3:73 and know I can do a thick cut ring and pinion or change the carrier, just don't know if the rears are the same.

[sigshooter]

Ring gear/pinion and the carrier are interchangable on GM 12 bolt trucks from 65 all the way up thru the 80's. I have changed my old trucks over to 3:40's from 70's and 80's 12 bolts.... no problems. Be sure and change the carrier also ( as you stated ) since there are different thickness of ring gears between gear ratios. Mr Gasket also makes a ring gear spacer ( which I have also used before ) that includes longer bolts if you didn't have the correct carrier

[Envy]

do you have the 3.08 POSI carrier?

[LT1 Burb]

I do have a posi carrier now with 3:08 gears but have a 700R4 so my motor lugs at freeway speed.

[Envy]

I have the posi with 3.73 's and i wanted to go 3.08. Id have to double check everything when i go to the shop next sunday but maybe we could work something out.

[likaroc13]

Originally Posted by LT1 Burb

I do have a posi carrier now with 3:08 gears but have a 700R4 so my motor lugs at freeway speed.

interesting...this came up in the thread recently regarding a 700R4 swap...i had mentioned being interested in doing the swap as well...however, about a year or so ago we refreshened & swapped a '72 rear with 3.08's into my '66 C-10, which has a Powerglide currently....the idea at the time was to help with highway driving....so, does this mean i'm gonna have to switch to a different gear set in order to freeway/highway drive with the 700r4?

[LT1 Burb]

I have fuel injected motor in my truck and it does pretty good at 19MPG but I have to cruise at 73 on the freeway to do 1900 RPM. I have 31" tires and I can watch my vacuum gauge and when I need to accelerate at 65 it really drops into the power mode to accelerate, yet if I drop down to 3rd it stays in the green when I need to speed up. It is all about tire and gearing and I think I can actually improve my performance and mileage by increasing cruising rpm's. I only turn 1400 RPM at 65.



I would do the swap but I will use this posi and gear set for my powerglide truck as it is an open carrier, thanks though.

[Captainfab]

Yep, that is a common misconception that lower rpm's means better mpg's. Not always so. The engine needs to operate in it's power band to be efficient. IMO 3.73's or 3.42's are a ideal gears for use with a OD trans. But that also depends on tire diameter.

[likaroc13]

Originally Posted by LT1 Burb

I have fuel injected motor in my truck and it does pretty good at 19MPG but I have to cruise at 73 on the freeway to do 1900 RPM. I have 31" tires and I can watch my vacuum gauge and when I need to accelerate at 65 it really drops into the power mode to accelerate, yet if I drop down to 3rd it stays in the green when I need to speed up. It is all about tire and gearing and I think I can actually improve my performance and mileage by increasing cruising rpm's. I only turn 1400 RPM at 65.



I would do the swap but I will use this posi and gear set for my powerglide truck as it is an open carrier, thanks though.

thanks...fuel injection definitely makes a difference i'm sure...i have a carbed 327...also, not sure if that last line was directed to me but i wasn't asking for your posi/gear set...i was just curious about the swap from Powerglide to 700r4, considering the 3.08's i currently have

Originally Posted by Captainfab

Yep, that is a common misconception that lower rpm's means better mpg's. Not always so. The engine needs to operate in it's power band to be efficient. IMO 3.73's or 3.42's are a ideal gears for use with a OD trans. But that also depends on tire diameter.

yeah, i've read that as well...and i've also read of a couple folks with a 700r4 w/ 3.08's....my truck has a 327, Edelbrock Performer intake, Edelbrock 600 carb, and otherwise pretty stock i think....current tires are 205/75/15, which is at 27.10 overall diameter

[LT1 Burb]

Well, I won the posi carrier and gear set from ebay so hopefully this really brings my truck to life.Looking at increased performance and maybe a big smoky burnout once in awhile.

[TopFuel1966]

Originally Posted by likaroc13

interesting...this came up in the thread recently regarding a 700R4 swap...i had mentioned being interested in doing the swap as well...however, about a year or so ago we refreshened & swapped a '72 rear with 3.08's into my '66 C-10, which has a Powerglide currently....the idea at the time was to help with highway driving....so, does this mean i'm gonna have to switch to a different gear set in order to freeway/highway drive with the 700r4?

I have a 3:50 gear behind a 4L60E and I definitely would not be able to stay within the power band with anything higher than that.

[LT1 Burb]

Yeah it really sucks because I am forced to always downshift, hopefully this helps with acceleration, mileage and driveability.

[Greshampiglet]

My 64' Stepside I have 3.54's and a 700R4 and a goodwrencg gas 350 I get 18 to 20mpg on the freeway 15" tires runs right about 19 to 20 RPM's around town I drive in 3rd I love it

[dracko]

Did 3.54's come stock in these trucks? I always assumed my truck was 3.73 because that is the most common. Maybe i'll have to do a tooth count and check.

[gimmy 64]

3::54 were dana 44 found in the GMC

[padresag]

http://www.billstruckshop.com/60-66used.html
has a 3.08 and a 3.73 posi listed for sale 1st column down
ron