Hey I just bought my first car which happens to be a 1988 Trans am GTA 305 TPI with the 700R4 and I was wondering what size the rear end is in it. Any help please?

 

Probably the 7.75 Borg Warner 9 bolt. IT could be the 7.5 GM 10 bolt. Count the bolts in the cover, or look for a rubber fill plug in the cover.

 

It has the rubber fill plug right on the back. The reason I want to know is because I want to put 3.73 gears in it.

 

That is the 9 bolt. It will be a 3.70 gear. Be prepared, they all make a little noise.

 

It has a 3.70 in it now? And what do you mean by it makes a little noise?

 

If it has 3.70 in it now then they're not original and going from 3.70 to 3.73 won't do anything except make your wallet lighter.

According to the database, a 1988 GTA with a 305 only had a manual transmission and the 3.45 geared 9 bolt was the G92 option. The 350 got the 700R4 and came with a 9 bolt and 2.77 gears or 3.27 with the G92 option. 2.77 gears is a 2 series carrier and not really upgradeable. 3.27 gears is a 3 series and can get the deeper gears installed.

Good luck finding inexpensive parts for the 9 bolt. Australian made with very little aftermarket support.

 

I don't know what gears it has in it. That's what I want to figure out. It has the 305 TPI with the automatic

 

As stated above

If you look at this chart you will see that the 350 came with a 700r4 Transmission.

The 5.0 came with a Manual Transmission.

https://www.thirdgen.org/1988-pontiac-firebird

The possible gear ratios are there also.
If you have the (G92) option than you will have the higher gears.

Those ratios are not absolute because they could have been changed.

 

My car is a 1988, the 8th digit is an F which is a 305 and I have an automatic and it is the GTA so what would it be? I'm not saying you're wrong, I'm just confused now. And I don't know if the gears are changed, but what would they be if they were stock?

 

My 88 GTA has a 305 TPI, 700r4, 3:27 rear gear, stock setup.

 

Thank you scoflaw. That really helps.

 

No, it doesn't have a 3.70 in it now. The gear that you get will be a 3.70, not a 3.73. Almost all of the 3.70 gears make noise. They usually only do it in a 5 MPH range, which is around 50 to 55 MPH in my experience. Just don't expect them to be quiet.

If you have a 3.27 in it now then you can use the 3.70 gears. If you have a 2.77 in it now then you will have to get a 3 series differential before you can use the 3.70.

 

Don't forget to add the Limited Slip additive, about a 6 ounce tube, to the diff when you fill it. If you don't you'll have clutch pack chatter at low speeds.

Found that out about 20 years ago when I thought my Aussie diff was toast. Added it and problem was solved.

 

Thank you for the info. And do you know why the 3.70 makes weird noises in that 5 mph range?

 

It's the way the gears are cut. Just something about them causes them to make a little noise. All of them may not do it, but most do.

 

Oh okay. Thank you.

 

Without having an rpo list how would you know if you have a g92 rear end option?

 

Pull the cover and look inside the rear

 

Jack up the rear axle. Put a piece of tape on the driveshaft and a piece of tape on the wheel. Turn the wheel around 10 times and count how many times the driveshaft rotates.

If it's a posi rear axle then both tires will turn together same direction.
If it's not a posi rear axle then tires will spin opposite directions and you need to multiply your numbers by 2 to get the gear ratio.

 

I haven't taken the cover off. But this tag is on it. 3.27 gears with disc brakes. It has a fill plug on the right side of the cover looks kind of like the differential on my wj grand cherokee I used to have.

 

Problem of course is, "G92" isn't "a gear ratio" on its own, or anything fixed, like that. It means "upgraded gear ratio". Obviously the exact meaning of "upgraded" has 2 components: what the "standard" one was, and what the "upgraded" one is.

In some years/models of the 9-bolt, the "standard" was 2.77, and G92 got you a 3.27. In others, "standard" was 3.08 or 3.27, and G92 got you a 3.45 or 3.70. So a 3.27 could be either, depending on the other details of the car. (engine & trans)

G92 also usually came with the dual cat setup, at least in some years. That gives you a possible way to tell. If your car has that, it probably came with G92, but by no means certain; if it doesn't, who knows, as it could have been changed out at some point.

But all that aside, what difference does it make if the car came with G92 or not? What REALLY matters is THE PARTS it's built out of NOW (which might not be the same as it came with anyway); such as, the gear ratio. If you want to know what the gear ratio is, far better to proceed directly to determining the gear ratio, rather than worrying about RPO codes.

Personally if it was THAT important TO ME, I'd just make a point of changing out the fluid, since it needs that done anyway; and accidentally count the teeth while I had it open. I can take one look at the rear in the photo and see that it needs new fluid, in any case. Just like they all do if YOU haven't done it. Not much point in posting on the Interwebz about something you can find out yourself by catching up on the deferred maintenance the car needs.

 

the crossroad I'm at is the amount of fabrication and modification I'm ready to do to this car will basically take everything that makes it a gta and throw it away so if someone actually wanted a tpi 350 gta that was all original then it would be more beneficial for me and them to sell this car and buy a roller with a decent body, instead of killing off another decent and compete car. I've always thought 3rd gen cars were cool looking but mechanically I didn't know much about them because I grew up on mk3 supras and 4runners.
I know toyota overbuilds their vehicles to ensure they can last so my thought process is take out all the weak links and build it out. But if I'm going to make a decision to keep or sell this thing I need to know what it has and will it last.

 

F-body's are the opposite of that.

 

lol. I'm a fan of underdogs and sleepers