posi rear end

[macdodd]

How do I determine which is the right posi rear end for my truck,,,I heard you can pull the rear end cover and count the teeth on the spline? Thanks for any input,,,,,Bill

[stepsider]

Count the ring gear teeth & the pinion teeth. Divide the pinion number into the ring gear & you will have the ratio. Beat gear ratio depends on your engine & tranny combination. Do you want gas milage or performance.

[macdodd]

I'm running a 350 with 305 heads,,,3 speed auto trans,,,,,just checked the mph, got around 15mpg at around 65mph. Thinking about changing to a 400t/r trans later on,,,but for right now,,,its running good. Just like to have a posi in the back,,,,I've found some posi rearends online,,,,,just don't know which ones will fit mine,,,also depends on how many bolts on the rearend cover?

[boraxman]

For a typical C10 you want a 12 bolt truck rear end, keep in mind there are two different overall sizes (drum to drum) so you will want to probably stay with the same width, I think the difference is like 3/4 inch. 67-70 narrow and some 70's-72 wider.

Then you will want to verify if theres posi in it or not along with the gear ratio. You can also add your own posi, if you find one without. Have have professional do it if you go that route

Confused yet? I am/

Im sure some others will elaborate....

[macdodd]

The guys that have looked at the rearend,,say its the orignal one. It doesn't have one in it,,,so can I put the posi lockup in the housing,? I've got a guy that'll do,,,,right now,,,its two turns of the driveshaft, to 1 complete turn of the rear wheel,,,

[cdowns]

if u switch to a turbo 400 tranny you'll get less mpg and loose power thru the hole range

[macdodd]

At 70mph,,,I'm running around 3k,,,we're thinking of going up to the Maggie Valley area in fall,,,not on the interstate all the way,,,but just don't want to push the engine,,,its running good right now,,shifts good. I'm gonna have a trans. guy I know take a look at it and check the shifting modes,,,I'm sure its been quite awhile since it was checked,,,1st gear shifts really quick,,unless of course,,,I get in it,,,,LOL,,,

[Stocker]

^^ Cliff nailed it. TH400 will rob power which means you'll burn more fuel. No advantage to it over a TH350 when you're running a typical SBC. TH400 was designed to stand up to the higher torque loads of a BBC. Neither trans has an overdrive, and final gearing is the same.

[macdodd]

With the 400t or r have an overdrive,,,,which might help the mpg?

[Goose]

I just had an Auburn posi unit installed in my '72 GMC 12 bolt. I kept the stock 3.07 gear ratio because I still have the Turbo 350. I turn about 3000rpm at 70 so you may have the same or similar ratio. If and when I can afford the overdrive upgrade (700R4) I'll have the gear ratio changed to 3.73.

[macdodd]

Did you buy the posi unit new or used,,,,and if, not being too nosy,,,what was the price and how hard to install?

[chevytruckluver]

I have attached 3 pictures for you the first is of an open (non posi)rear axle
the second is a picture of one of the gov lock style carrier. (stay away from this type)
The 3rd is the Eaton style which can be rebuilt of you were to buy a used carrier. If you start to look for a used carrier It must be from a truck not a car. ( there are 12 bolt car and 12 bolt truck rear axles).
The first one is what most all of our trucks came with. Also most of these trucks have a 308 gear ratio.
It is not an easy task to change the rear gears or the carrier unit as it takes special tools and requires lost of measuring. If its not set up and installed correctly it will cause premature gear wear and gear noise.
Good luck

[hdff]

Originally Posted by Goose

I just had an Auburn posi unit installed in my '72 GMC 12 bolt. I kept the stock 3.07 gear ratio because I still have the Turbo 350. I turn about 3000rpm at 70 so you may have the same or similar ratio. If and when I can afford the overdrive upgrade (700R4) I'll have the gear ratio changed to 3.73.

I have a 700r4 and just had a eaton posi installed, I also changed the gear from a 3.08 to a 3.73. I am happy with it except I need to bump up the power on the 402 a bit... Lol set me back about 1400 with a new yoke and seals

[macdodd]

Thanks for the pics,,,,I have a mechanic that I deal with,,,so I'll have him do it or find a speed shop or rod shop that knows about changing it out,,,,thanks for the info,,,,Bill

[cdowns]

if you purchase the new posi and gear set as well as the install set you should b good to go with labor in the 1200-1500 range depending how good a deal you get