Hello guys
I noticed my car runs kinda fast compared to the 2.73 rear end ones, infact I checked my speed/RPM when cruising and I discovered I could have 3.xx rear end. What do you think? That's my car running 70mph in 4th gear with NO lockup. My car is an early 87 Trans Am 305 TPI full loaded

 

Check the accuracy of the "tach" first.

The stock tach in these cars is well and widely known to fail, by reading high. Sometimes WAY WAY WAY WAY high; like, I have one that reads TWICE the real RPM. After all, it's not really an "instrument"; it's a "sport trim package".

Meanwhile, if G92 is in your RPO codes, the car came with it; if not, it didn't.

If it REALLY matters to you what gear you have, then it also matters that you'll want to keep it in good shape. Great place to start would be, by changing the fluid, which obviously you have never done. Start there, and while taking care of your long overdue "deferred maintenance", make a point to LOOK AT the gears. What you see, is what you have.

 

In 1987 I don't believe the G92 gear was available without the J65 4 wheel disc brakes & G80 Posi rear.....and those didn't come on a 10 bolt.

 

I do not have the RPO sticker and the rear fluid has been changed yesterday with 75W90 and posi LSD additive from my mech,so I was unable to read on the gears!
For sure I have a g80 RPO and my gauges are stock!

 

RU6 I believe is the code Firebird / Trans Am equivalent to G92 for Camaro's....

 

RP6??? Not until 91 & 92.

 

Thing is who knows who done what to the car before you got it. Jack it up, put a mark on driveshaft, put car in neutral, and start turning the wheel and count the revolutions of the wheel vs the driveshaft it will get you close. If it turns 3 and a lil then you have 3.08 or 3.23. Im pretty sure stock you should have 2.73

 

Snap a pic of the SPID in the console and we'll tell you what gear your car came with.

 

I do not have the sticker for RPOs so the only way is to check the speedo with a GPS and see if readings are right