How to find out what gears I have?????

[Uno99]

I would like to do a gear swap but need to know what is in there from factory. The truck was my uncles and he cannot recall what they are and he never messed with them.  The doors still have stickers on them is there a code I can look for on it??  The truck is a 81 GMC High Sierra and used to have the 5.7 desiel in it. It still has the original th 350 and rearend too.

[Blazin]

The only sticker that has that info is inside the glove box. Other than that you will need to pull the diff. cover and either count the teeth or divide the last two numbers on the ring gear into one another.

[Captkaos]

There are also codes on the axle tube, but I don't have the book to decipher them...  Best way is to pull the cover.

[Uno99]

Well I looked in the glove box and found some info:  It says rear diff STANDARD then and option code of GQ1?  Anyone know what GQ1 stands for.

[Fordeatinz71]

lol, i had that retchid code too.  what it means is, you have to figure out what the standard gear offering was for that EXACT truck.  looks like your standard ratio is the same as mine i'm sad to report...2.73's.  unfortunately for me though, mine is in a 4x4 with a 305...

[Captkaos]

On an 81, it is either 2.73 or 3.08.

[Uno99]

OK here are a few more questions.
1.  So if I find out they are 2.73 or 3.08 the highest i can go without changing the carrier is 3.42 right?
2.  What size is the rear in these? GM 8.5, 8.875 7.5, 7.625?
3.  What is the pinion spline count as well?

Thx for the help.

[Captkaos]

1) Yes, being a 81 I would think it has 2.73's (could be wrong).  3.42 is max on the carrier unless you get a spacer.
2) 10bolt = 8.5, 12bolt = 8.875
3) It should be 28 spline.

[81Cheyenne]

Just curious, is it possible to tell by the speedo connection on the transmission?

[Captkaos]

Tell what by the speedo connection?

[Fordeatinz71]

the gear ratio.  and honestly, yea, you could.  you could pull out the original (i'm assuming it would be original...if its ever been changed to compensate for bigger tires then it won't tell you anything) speedo gear and figure out what color it is and how many teeth it has and then have your local stealership cross-reference the info and tell ya what gear ratio/transmission it should be paired with.  but like i said, if your transmission has been changed to something else than what the truck came with or the gear was ever changed to compensate for a tire change then it wouldn't tell you anything.  you have to be absolutely POSITIVE that your truck is all original.  it can be done, but it's not really a "reliable" way of telling since so many of these trucks were modified...

[Captkaos]

I listed the colors in another post somewhere in here, but the easiest is just pull the cover.  I am sure the fluid hasn't been changed in a while.

[Uno99]

I think 3.42 with a th 350 is steep enough for me.  I swapped in 3.73 on my 99 Trans Am and even with OD it whines out on th highway more than I like.

[sav_x69]

the number off my carrier is 1 GM 14012787 12 41 8 82  can someone figure it out for me because i cant make any sense of it  rear end is 83 2500 4x4 14 bolt any help appreciated

[Fordeatinz71]

well, i don't have the interchange manual to run the part number and can't remember exactly how to decipher the numbers...but i'm sure someone will chime in with the right info...