G.m. 12 bolt truck & car differences

[NewEnglandRaceFan]

Hello I have a 2nd 12 bolt out of a 67-72 c-10 that once the snow melts and uncovers it. I'll open up to see what is in it..

are the truck carrier splits(gear) the same as the cars..?
how do you tell what carrier you have, if no gears are on it. like when looking at them at a swap meet or ebay add?

do they make a spacer to replace the crush sleave like they do for the 10 bolt.. and is it wise to use them..??


The one under truck is 3.08 open and when I install the 2004r I think that might be a tad tall.. has 26.4" tall tires on it now (295/50/15) but may go to 28" tall tires... if when I get bed back on.. the 26" tall still look small even with the 2.75" drop

MY 86 monte SS with 2004r/373 and 26.1" tall (215/65/15) is like 2200
the 3.08 with 26" tire is about 1800 rpm at 65mph
28" tire is about 1600 at 65mph
28"tire with 3.73 is 2000 rpm at 66.66 rpm..

I have no bones about not using o/d until 65-70mph..

but would like to know what the lowest (highest #) gear I can slap on my carrier without spacers.. if I go that route.. but if I'm pulling the gears I might as well put a posi in it.. and depending on what I find in the 2nd rear.. will help determine what happens with the carrier part of this..

engine is a 350 vortec with ramjet upgrades.. and may down the line find a 489 in it.. with oval ports.. for low end (might even try the unloved peanut ports )

links/ info/etc big help..
I'm sure it's been gone over a bazzionon times.. and I've saved about 10 threads to read over later... but why not one more..

P.S. Keep in mind this truck will be used for LSR(land speed racing) and street driving .. but gearing will be more so for the LSR unless I install the ford truck 9" I have and 4:11 gears for street and ? for the LSR

[Chad's Dad]

The car 12 bolt and truck 12 bolt are completely different units. It's hard to believe but the truck 12 bolt has a smaller diameter pinion which makes it the weaker unit of the two. The truck unit is not the best differential to spend your money on. I would recommend a car 12 bolt or a 9 inch ford.

[squirrel]

sounds like it's not a vehicle that needs the toughest rearend available, the original rear should be fine.

3.40 and 3.73 are different, so you can only go up to 3.40.

[VTJUNK]

Car ring and pinions, differentials can be used in truck housings by juggling bearing sizes. If you want to go 12 bolt passenger car, how much wider is an Impala 12 bolt? Those are more common (cheaper)

I have a 3.73 gear under my 71 C10. Too much gear with the 454/th400, it turns 3,000rpm at 65. I'd consider swapping my 3.73 for your 3.08.

Been looking for a complete 3.42 rear to swap but haven't found one. Since I use the truck primarily for pulling the car trailer, I'm getting sick of 9mpg. like to do a swap to a 5.3/4l60/3.42 or at least a high torque 350/th400/3.42

[NewEnglandRaceFan]

if I get to swap'n axle's I'll just use the f150 9" I have here.. I have one pumpkin with new 4:11 and alburn l/s .and whatever the pumpkin in the rear has... rather just use the one under the truck and maybe change the gears.. hope'n to get lucky and the other 12 bolt is a posi and 340 gears.. if not.. then it'll become what is more economical.. maybe just stuff a lockrite in the 308 open well see.. thanks for the info big help.. need to get printer up and running so I can print out stuff and put it in a 3 ring binder.. as I'm sure some stuff I've asked before.. and just forgot..

[NewEnglandRaceFan]

Car ring and pinions, differentials can be used in truck housings by juggling bearing sizes. If you want to go 12 bolt passenger car, how much wider is an Impala 12 bolt? Those are more common (cheaper)

I have a 3.73 gear under my 71 C10. Too much gear with the 454/th400, it turns 3,000rpm at 65. I'd consider swapping my 3.73 for your 3.08.

Been looking for a complete 3.42 rear to swap but haven't found one. Since I use the truck primarily for pulling the car trailer, I'm getting sick of 9mpg. like to do a swap to a 5.3/4l60/3.42 or at least a high torque 350/th400/3.42

might take you up on that.. but I'll see whats in my spare rear. if it's a 3.08 they are yours.. no swap needed.. still playing with gears and tire sizes to get an idea what would be ideal.. for not only o/d but for all around driving.. we are getting 4 days of 40+* so hopefully the rear will be unburied enough from the snow that I can pop the cover and take a look at what's in it.. I got it at an "all you can carry" week end at my local junkyard.. 35 bucks all you can carry..iirc the truck had a 327 in it so I'm thinking 67-68 truck. I put that bad boy on my shoulder and walked it up to the counter.. they guy was like you are nuts.. I said I'll be back after I drop this at home.. came back and he let me in without paying again WIN.. that trip was bear hugging a sbc block empty.. with 2 sanderson a/c compressors roped over my shoulders... The owner wouldn't let me back in that week end.. after that.. oh well there is always next time..

[squirrel]

I had a 66 suburban, 3.73 12 bolt truck rear, and 454/th400. I found a mid 70s van rear with 3.07 gears, and swapped the innards into my suburban. made it nice on the road.

I have a 3.08 10 bolt in my 57 suburban now behind the 350/th400. Seems to work ok. No tach, so I don't care how fast the motor spins.