So ik this is for third gen stuff but i drive a tbi truck and you guys know alot about the too. Anyway my neighbor had a square body 80s era chevy truck its only good as a parts trck and he may let me buy the rearend and motor out of it for a future project but i would want to put the rearend in my 90 truck. My question is how do i determin the readend ratio? Where is the pro code in my truck its in the glove box but this one seemed to not be there i looked and it had a blue sticker with no codes on it and its a 4x4 and the trearend was 6 lug so whats up with that? I don't know much about a 6 lug rearend. Any ideas?

 

Pop the cover & look at the gears.

 

6 bolt axles is normally referred to as a heavy half. Still a half ton suspension. Probably just a 8.5" 10 bolt diff. Might be a Dana.

Try doing some research on truck forums that cater to 80's GM pickups since this has nothing to do with third gens.

 

1/2 ton 4 X 4 Square bodys came with 6 lug axles all the way from 74 to 87. 80's model trucks came with what was referred to as a corporate 10 bolt. Not quite as strong as a 12 bolt, but stronger than a passenger car 10 bolt. 2WD trucks had 5 lug axles.
Axle ratios can determined buy rotating axle one revolution and count pinion yoke revolutions. Not exact, but will get you in the ballpark.

 

well the truck is sitting on blocks and its been there a long time ik you can spin the rearend i just wondered about the pro codes cuz its way easier to read a code then jack the thing up ya know? I honestly just wondered if it was worth swapping over and doing all that in my truck. I would need 2 more wheels tho to make it work. I was looking at it to be a cheap posi thing but i would have to do alot of work to the break system on it im sure. That is if the rearend was not locked up from sitting so long,

 

you could prolly just swap your 5 lug axles into that housing to make it work

the RPO codes are all the same in regards to gear ratios, no matter if it was in a truck, wagon or corvette.

So look at the rpo codes and start googling them. Here is a quick list I found on fullsizechevy.com

GT4 -- AXLE REAR, 3.73 RATIO
GT5 -- AXLE REAR, 4.10 RATIO
GU4 -- AXLE REAR, 3.08 RATIO
GU5 -- AXLE REAR, 3.23 RATIO
GU6 -- AXLE REAR, 3.42 RATIO
G80 -- AXLE REAR, LIMITED SLIP (POSITRACTION)

HC4 -- AXLE REAR, 4.56 RATIO

 

I'm surprised somebody else 'sofa' hasn't already chimed in on this, but RPO codes don't mean **** if somebody has been in there tinkering with it sometime in the last 30 or so years. Only way to know for sure is to pop that cover & take a look.

 

Well the truck looks stock and the owners told me theres never neen any major work done too it so i expect the rearend to be untouched.

 

Hey thanks, yeah im familiar with pro codes and i know most of them, atleast alot of the rearend ones. Will my c1500 axles fit into that old housing? I can probly get gone with it for 250 and it i can do that clean it up and swap my axles that would be great.

 

You *might* get away with that.....

but you would have to pull the axles to find out. At some point, GM switched from 26 spline axles to 28 spline, which I *think* was 87/88 in the fbody. Not certain about the trucks though

and yes, someone could have swapped the rear. Knowing the codes is just some knowledge. If you walk up and it has a code for a non posi 2.73 rear, I would walk away.....

 

yeah i already have a 3.42 non posi i was hoping this truck being a 4x4 heavy half was a low axle ratio truck but i haven't gotten to go and look it didn't have the white procode stickerjust a blue sticker so the pro codes may be gone

 

You could try to look up the VIN, that would get you the RPO codes.