12 bolt truck rear.

[epfatboy]

I’m going to intall some 3.42 gears and a Eaton posi unit in my ‘73 C10 to support my 4.8 turbo build. Pulled the cover and found a good clean open 3.73. Has me wondering if I should keep the 3.73 and Just run a taller tire!

Are these 12 bolts all 30 spline? Think I can get enough of a look at the axle splines if I pull the pin and shove the tire in?.... being lazy!

[sweetk30]

with the limited info you provided i played with 2 options .

with say a 4l60-e in overdrive @ 70 mph & 28" tire .
3.42 = 2011 rpm
3.73 = 2193 rpm

then th350 for non overdrive @ 70 mph & 28" tire .
3.42 = 3035 rpm
3.73 = 3310 rpm

the overdrive #'s are in the sweet spot for most who want to drive and cruise . the non overdrive #'s are up there . . .

here play with this and get your combo in and scroll to the bottom for rpm results . and you can change just about any # you want in this calculator by clicking it . http://www.grimmjeeper.com/gears.html

[Matt Man]

Originally Posted by epfatboy

I’m going to intall some 3.42 gears and a Eaton posi unit in my ‘73 C10 to support my 4.8 turbo build. Pulled the cover and found a good clean open 3.73. Has me wondering if I should keep the 3.73 and Just run a taller tire!

Are these 12 bolts all 30 spline? Think I can get enough of a look at the axle splines if I pull the pin and shove the tire in?.... being lazy!

12 bolt truck rear ends are 30 spline, with a smaller pinion than a 10 bolt rear end but the pinion usually doesn't break. The ford 9" rear has a even smaller pinion than the 12 bolt! Great axle that was used up to the 1981 model year, my 81 k10 has a 12 bolt rear with 4.10 gears and a Detroit locker, great setup.

[epfatboy]

Yeah sorry about that.

Tranny is a 4L80E, the current tire is a 315/35R20 so its just a little over 28.5" tall.

Not worried about the pinion, it's the axles that let go first and honestly with a street tire I don't how I'd twist a 30 spline axle.

Seems like I should stick with the 3.73 and see what happens given the lack of difference in RPM. I know most of the turbo guys like a 3.42 but I'll give this a try and see how I like it. Plus keeping the gears will make adding the posi unit a lot easier.

[sweetk30]

@ 70 mph = 2294 rpm 3.73 gear.

@ 70 mph = 2104 rpm 3.42 gear.

reason is most likely some motor combos pull better with a bit of a drag load on the system . my 1 atv if i swap in a smaller engine sprocket the motor just winds up . if i go 1 tooth up in sprocket size on the engine she PULLS like a freight train

[epfatboy]

Originally Posted by sweetk30

@ 70 mph = 2294 rpm 3.73 gear.

@ 70 mph = 2104 rpm 3.42 gear.

reason is most likely some motor combos pull better with a bit of a drag load on the system . my 1 atv if i swap in a smaller engine sprocket the motor just winds up . if i go 1 tooth up in sprocket size on the engine she PULLS like a freight train

Right and with an FI motor that is what I’m worried about. Just posted a question in the LS FI forum to see what I get.

Is there a decent site where I can get GM gears for this rear? Not a fan of Richmond for DD’s they can make noise

[andyh1956]

My son ran a 3.42 against his LS & he was planning to install 3.90s before it got T-Boned...
The engine was 500 rpm off its low end curve w/ 3.42s & he had a 5.7, your 4.8 would want better than 3.42 s also I would bet..