12 Bolt rear ends

[customizer76]

Could someone explain to me why you can find a 3:90 gear for the 12 bolt car rear but not the truck. I would like a little more than 3:73 but not a 4:11 for the rear of my 79 C10 but not able to find what i'm looking for. Can my chevy community help me out here.
The other option is to do the 4:11 with and overdrive tranny. 700R4 or 200R4 which one to run?
Help me out friends!!!!!!!!!!
One more thing that maybe helpful i'm getting 450 to 475 @ the flywheel

[68 TT]

Originally Posted by customizer76

Could someone explain to me why you can find a 3:90 gear for the 12 bolt car rear but not the truck. I would like a little more than 3:73 but not a 4:11 for the rear of my 79 C10 but not able to find what i'm looking for. Can my chevy community help me out here.
The other option is to do the 4:11 with and overdrive tranny. 700R4 or 200R4 which one to run?
Help me out friends!!!!!!!!!!
One more thing that maybe helpful i'm getting 450 to 475 @ the flywheel

The car & truck gears are slightly different with the car version actually having a larger pinion bearing diameter making it stronger. Go figure. I wonder why they didn't just use the same car 12-bolt center section in both.

At 450 to 475 hp you will need to spend a decent amount of money on a 700R4 or the 200-4R overdrive trans to make it hold up for more than a few minutes of hard driving. They were never put in anything with more than 275 hp so they were not built to handle much more. The 4L80E is a better choice but takes a bit of work to get it fully functional in a non-computer controlled truck. There are manual shift versions out there that they are calling a 4L80 that require no computer to operate but you have no automatic shifting function, all shifts have to be done manually with the shifter. I'm considering going this route in my 68 Camaro.

You could always just run a shorter tire with the 3.73's or a taller one with the 4.11's and keep the trans you already have.

[LONGHAIR]

You are only looking at about 300 RPM or so difference at highway speed between 3.73 and 4.11 anyway.....so it's probably not worth the hassle to try to get between them.

[N2TRUX]

I would consider a built OD transmission. The lower first gear may give you what your looking for and certainly will give dividends on the top end.

[customizer76]

Thank you all for all the info. I figured the OD tranny would not be cheap. I Think I will just keep my TH350 and select one or the other on the gear end.

[kelgar50]

I went from 4.11's to 3.73's trying to drop some rpm's so I could get better mpg but still have some decent take off power but I did not see that much of a difference by switching in that small of a step.I wish I went with the 3.42's but the ither truck I had had a 700r4 with 3.73's and it had would pull hard with that setup......down side is back then the 700r4's were not built strong enough and I was tearing them up.
They have the bugs out of them now and depending who builds them they can live behind 500hp engines.The same can be said with the 200's as well I know people that have put them in there hot rods behind 354 and 392 hemi's and they are living as well.Also the 200 is the same size as the TH-350 so you don't have to cut the drive shaft down.

[68 TT]

Originally Posted by customizer76

Thank you all for all the info. I figured the OD tranny would not be cheap. I Think I will just keep my TH350 and select one or the other on the gear end.

You will be better off with the 3.73's and a T-350 on the street. You make enough power that the little bit of extra gear going to 4.11's isn't worth the extra RPM while cruising down the highway. You can push the steeper gear without needing to downshift on every little hill with the power you have.

They are pretty common gears in the truck 12-bolt too so you may be able to pick some up in decent used shape for cheap.

[Damien]

Originally Posted by kelgar50

Also the 200 is the same size as the TH-350 so you don't have to cut the drive shaft down.

The 700r4 is about the same length as a long shaft Th350 so the same drive shaft can be used.

[customizer76]

Thimk i have made up my mind now, 3:73 it is and thank you all again.

[78-5.3]

thats what im looking for 3.73

[Pyrotechnic]

Someone on here said they got the car gears to work, the trick was a different pinion bearing I think. Never found out the details about it though.

[68 TT]

Originally Posted by Pyrotechnic

Someone on here said they got the car gears to work, the trick was a different pinion bearing I think. Never found out the details about it though.

Maybe the pinion bearing outer diameter is the same between the two. I know the pinion itself has a smaller spline count on the truck 12-bolt gears.

[68 TT]

I will play around with it this summer when I am tinkering on the C-10 and see if I can get the car 12-bolt carrier & gears to fit the truck 12-bolt.