Contemplating putting either 38's or 40's on my 1990 K5.
Do you think the stock 10 bolt in the rear and GM corporate up front
would be able to take the strain? Or should I go hunting for Dana 60's
off 1 tons? Truck will just be used on the pavement and mud, no technical rock climbing where the axles will be under lots of stress.

 

its such a gray area. some people will say heck no and some people will say heck yes.

it depends what gears you run also. if you put some low gears lilke 4.56s in there the teeth are gonna be so small i hear people breaking them running 35s. i ran 38/12.50 tsl swampers for about 7 years on 10 bolts with 373 gearing. the axles have outlasted 3 trucks. they were even under a 6000lb suburban with a 400ci engine. had these axles since about 2002-2003 and they are still in use under my s10 with 33inch boggers. spider gears are chipped up and the front carrier is a little busted up but still holding together all this time later

basically you have to remember you have 10 bolts. if you dump it on asphalt in 4low with a big block yeah youre gonna break them, if you ease on and off the throttle in the mud, shouldnt be a problem

 

thanks for the info man

 

35-37" tire is the breaking point in my opinion for upgrading axles.

while you don't need a D60 up front if you're going to dump $1200 into a gear swap why not just buy a D60 and GM14 that already have the 4.56's or even better 5.13's and spend the extra $500.... 4.56 gears in a 10bolt is not going to last long on 40" tires and not to mention that it will still be way undergeared.

I replaced 3.73's with 4.56 and the speedo is dead-nuts with 37x13.5 tires. Do some fancy calculations and you'll see that the increased weight of the tires rotational mass is not offset by the gears and I'm still relatively too high a gear.

 

Absolutely change the rear to a 14 bolt if you're switching to anything 35" or taller. If you're doing a lot of freeway/highway driving, you'll want to stay with a taller gear, i.e. 4:11 for 35's or 4:56 for 37-40s. If you've still kept the 700r4-208 combo, you'll want to gear it so that your RPMs are about 2k highway cruising speed when it goes into lock-up.