I've been doing some research on the rearend in my 83 Z.
My father in-law, who is the original owner, said he ordered the car with a 4:10 posi. So I looked at the code stamped on the passenger side axle tube and it looks like 5JK. I'm not sure what the 5 is for, but in my research JK is supposed to be a 3:73 open diff. However, mine is definitely a posi.
Did I read the code wrong?
Maybe it is JR? Which is a 3:42 posi.
​​​​If so, the car tachs 2300rpm at 60. Is that right for a 3:42 gear?
The car has never been modified with the exception of the wheels, eibach sport springs and a mini high torque starter that I put on it.

 

I don't know about the 4/6 cylinder 3rd gens but the numerically highest axel ratio you could get was a 3.70 to 1 in a Borg Warner 9-bolt axel 1985/1986 and a 3.73 to 1 in a Saginaw 10-bolt axel 1983 to 1986.

As best I know you could only get those gear ratios in a car equipped with the L69 5.0L H.O. V8 engine.

EDIT: I was wrong. For 1983 the 3.73 was standard with the L69 5-speed. LG4 was standard with the 3.23 automatic or 5-speed and optional (G92) with the 3.73 automatic or 5-speed.

 

This is a LG4 car with a 5spd and 10 bolt rearend.

 

I never really looked into the codes on the axle itself. Your best bet is the look at your RPO codes. Another option is to plug in the information you do have into one of those online rear gear calculators. Calculate RPM for Given Speed(MPH), Rear Gear Ratio, and Trans Gear Ratio You can plug in your speed, rpm, tire diameter (I'm guessing not stock size tire if you changed wheels) and your final drive of your trans to get your axle. For example 60 mph at 2300rpm and if you had stock 215-65-15 tires makes them 26" tall with the 700r4 that has a 0.70 final gear that would put your rear gear around the 4.10 mark

 

Thanks Scotty.
I just did the calculations. My 5spd final drive is .74 and my rear tires are 25in, 255/40/17. So 2300 at 60 calculates out to 3:73. That tells me code JK is correct, but it's a posi and not a open diff as the code JK suggests.

 

I am glad I could help. This type of stuff is key to forums like this staying alive. I use gear calculators like that all the time for my wheel and tire selections,

 

I agree
Yeah, I use the tire calculators anytime I'm ordering tires and rims and measure the back spacing to know what I can fit in the wheel well.
This is my first time using the gear calculator! Much appreciated!

 

Is G92 an option for your car?...

 

I dont have the build sheet, so I'm not sure.

 

If you can find your RPO sticker, that'll tell you all of the options that would have come from factory. I believe in early thirdgens, it would be on the underside of the center console lid, please correct me if I'm wrong on this. Later thirdgens had 'em on the inside of the storage lid in the trunk. A G80 code on the RPO sticker would indicate the posi, and G92 would have been for a performance axle ratio; not sure exactly which ratio on the '83, think gear codes changed meaning a bit over the years, but might be mistaken.... brief search of the forum is telling me that GT4 would indicate a 3.73 ratio, while GT5 or GT8 would be 4.10s. More complete list is at the end of THIS thread

 

To the EXTREMELY LIMITED extent I would ever trust ANY "codes" on ANYTHING, I'd trust ones stamped on an axle, LONG BEFORE ones in the RPO list. At least, the axle stampings move around with the axle. Still no guarantee that what they say is what's inside, butt they're at least one step closer to the parts than RPOs.

 

True! OP mentioned that his father-in-law was the original owner, and he knows what has/ hasn't been touched on the car, so RPOs oughta give a pretty good idea in this case at least. In general though, yeah, hard to know just how much about a car has changed after it's been passed around like a flask at homecoming

 

Thanks for the info LL! I'll trust the tire/gear calculator that it's a 3:73.
I don't want to risk damaging the console just to confirm what I already know. Without a doubt, I know it's a posi regardless of what the numbers say and I know for a fact it's never been modified because the first owner is my father in-law and I'm the second owner.

 

No worries! I was curious since you mentioned he said he ordered the car with 4.10s... when I say underside of the lid btw, I mean the part that faces into the console, should theoretically just be sittin there lookin at ya when you lift the lid open. If there isn't a sticker in there, they mighta put em somewhere else on '83s, not sure. In any case, sweet car, cool to see it stay in the family!

 

Thanks man! Yeah I've looked there, under both front seats and in the spare tire, cargo area behind the plastic trim. Nothing. 🤷‍♂️
Also, he had a 64 Corvette and maybe that car had a 4:10 and he's confusing the two.

 

JK Is Not Posi