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4.10’s or 4.56’s with 700r4, 35” tires

Just a rear .
By today’s prices that’s O.K. but better prices could be found.

If that axle is a CUCV 4.56 & Detroit Locker equipped than buy it today.

If the guy does not know have him pop the cover and take a few pics - we can identify for you.
 
Well it depends on the gear ratio your truck was equipped with, but in the 80's GM built these truck with gearing that had no business in a 4wd. For example 2.73.

Funny you mention that, the original drivetrain in my old '80 K10 was a 350 / 4spd / 205 (of course) and 2.73's! Needless to say, when I bought it with 38's I HAD to use granny gear to get moving because it was a gutless wonder. :yikes: :rotfl:
 
Here’s another one. 373’s, posi and still a 6 lug. $1000.

I’m certainly open to a 14 bolt….

14 bolt chevy posi rear axle
https://sfbay.craigslist.org/scz/pts/d/watsonville-14-bolt-chevy-posi-rear-axle/7768644413.html
That is a 9.5" ring gear, 14 Bolt Semi-Float and a $500 axle at best with the disc brakes. But it is a very good option for upgrading your rear axle without changing wheels and tires once you weld on new spring perches and shock mount.
 
Here’s another one. 373’s, posi and still a 6 lug. $1000.

I’m certainly open to a 14 bolt….

14 bolt chevy posi rear axle
https://sfbay.craigslist.org/scz/pts/d/watsonville-14-bolt-chevy-posi-rear-axle/7768644413.html

that is a SF semi-float. not a bad axle, already has disc brakes, not as heavy as the FF, doesn't hang as low. gear ratio is not optimal but that can be changed.
@Bent77 ran one in his truck with a doubler and wheeled it hard and it held up. I think it would be a good upgrade since you are going to run 35s. price may be a little high; just depends on what you are willing to spend
 
Funny you mention that, the original drivetrain in my old '80 K10 was a 350 / 4spd / 205 (of course) and 2.73's! Needless to say, when I bought it with 38's I HAD to use granny gear to get moving because it was a gutless wonder. :yikes: :rotfl:
John and I had a buddy who had a 81 K-10 with the almighty 305, TH350/208, 10B/12B with 2.56’s !
It was a pig with 31” tall tires goin down hill with a tail wind.

He immediately upgraded to 4.10’s and a rear L/S only to make another upgrade a year later to a 454 and standard issue CUCV 1-Tons with 4.56’s.
Feeling the difference between the motor and proper gear ratios you could not smack the smile off of his face.
 
that is a SF semi-float. not a bad axle, already has disc brakes, not as heavy as the FF, doesn't hang as low. gear ratio is not optimal but that can be changed.
@Bent77 ran one in his truck with a doubler and wheeled it hard and it held up. I think it would be a good upgrade since you are going to run 35s. price may be a little high; just depends on what you are willing to spend
Still in it
 
Still in it
IMHO, the S/F 9.5” 14B should have been standard issue in all the K5’s & 10’s - it’s a great compromise between size and strength.
Good ring gear and fat pinion size, 33-spline axles that get pretty fat at the ends for a decent size bearing and the carrier is big enough so that all the components have enough meat & beef to survive some added power and tire upgrades.
 
IMHO, the S/F 9.5” 14B should have been standard issue in all the K5’s & 10’s - it’s a great compromise between size and strength.
Good ring gear and fat pinion size, 33-spline axles that get pretty fat at the ends for a decent size bearing and the carrier is big enough so that all the components have enough meat & beef to survive some added power and tire upgrades.
Lot more beef than the 12 and especially 10b axles. Isn’t the CAFE standard for fuel economy adding weight though
 
IMO, 3.73 is not going to be a big enough change, so you'd still be looking at a gear swap. Are you set up to add spring pads and shock mounts at the correct angles? I can't really tell from the pics, but it kind of looks like the Summit discs have a parking brake, which increases the value. You can always make an offer. It sounds like you're not pressed for time.
 
Skipped all the comments but I have 315/75R16 tires (35s) with 4:10 gears and a 6.2L diesel and 700R4. It puts my speedo real close to stock. Plenty of takeoff low end with the deep first gear and a good place to go 65-75 down the highway. I get like 17 mpg with my diesel too.
 
Can’t tell you all how much I appreciate your responses. I think the general consensus is that 4:56’s should work pretty well for me.

At this point, I’m going to just put those into my 10 bolt, but am well aware that they may not last as long as I’d wish.

After looking at 5 in Ca, it’s a grand to me, more or less, to pick one up. Buying and rebuilding (regearing too) a used 14 bolt, I think would run me at $2500, which incentivizes me to take it easy on my 10 bolt!
 

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