When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
I don't have the build sticker for my car, so I am not 100% sure which rearend is in it. It is a limited slip, for sure. The car is a 1998 Trans Am A4 with the 150mph speedo. At 72 mph in 4th, I am at about 2000 RPM (maybe just a hair over). I am thinking 3.23 gears. These cars also came with 2.73 gears, so help me figure out which I have.
Those of you who know what rearend is in your car, what speed are you traveling at 2,000 RPM in 4th? (Autos only.)
2.73 had a 1st gear lockout-the inability to manually select 1st gear because of a plastic tab at the gear selector/indicator.
that tab must have been removed in the 273 bolt on car I used to own - it was fun to hold it sliding down the street against the rev limiter for a couple ticks before shifting lol.
2000 at 72 sounds like 273's but it also depends on the rollout/tire height if your not on stock tires.
another way to verify what the car came with is to check the door tag with all the broadcast numbers gu2+ 273 gu5=323 but if the rear has been swapped all bets are off.
depending which posi style it has you can jack it up and spin it to see what the ratio is but only if both wheels turn when you turn one , just see how many times the wheels turn to make one driveshaft rotation and you have the ratio.
At 72 mph in 4th, I am at about 2000 RPM (maybe just a hair over). I am thinking 3.23 gears.
I would agree that you likely have 3.23s based on this. If you had 2.73s, you should only be at about 1800rpm in 4th at that speed. This is assuming stock tire size and correct speedometer though.
Originally Posted by FirstYrLS1Z
Aluminum driveshaft=3.23
Steel driveshaft=2.73
That's another way to tell, assuming nobody has changed the drive shaft.
Originally Posted by FirstYrLS1Z
2.73 had a 1st gear lockout-the inability to manually select 1st gear because of a plastic tab at the gear selector/indicator.
This is not accurate for the LS1 years. There was no lockout present on any of the 2.73 cars from 1998 or beyond, regardless of gear ratio. That's an LT1-era only deal.
Originally Posted by murphinator
that tab must have been removed in the 273 bolt on car I used to own - it was fun to hold it sliding down the street against the rev limiter for a couple ticks before shifting lol.
As mentioned above, none of the LS1 cars had this.
Originally Posted by murphinator
2000 at 72 sounds like 273's but it also depends on the rollout/tire height if your not on stock tires.
another way to verify what the car came with is to check the door tag with all the broadcast numbers gu2+ 273 gu5=323 but if the rear has been swapped all bets are off.
OP mentioned that the sticker is gone. But he could certainly go to any GM dealer and get a VIS printout from the parts department for his VIN. This would show all the original RPOs, including GU2 or GU5 as you mentioned above.
Originally Posted by murphinator
depending which posi style it has you can jack it up and spin it to see what the ratio is but only if both wheels turn when you turn one , just see how many times the wheels turn to make one driveshaft rotation and you have the ratio.
It's actually the other way around (spin tire one full turn and count driveshaft rotations), but yes this method will work on a '98 since it would have the Auburn LSD (both wheels turn together.) The process is a bit different for an open rear or a Torsen style LSD - but neither of those would have come on a '98 V8 car.
Clarification on the 1st gear lock-out. Yes,it was an LT1 'thing' and as the OPs' vehicle is a LS1 Firebird,I should not have brought it up.
BUT,the 98 LS1 Z28 had it,discontinued with the 99 + later LS1 Z28s'.
Why GM had it for 98 LS1 Z28 and not the 98 LS1 Firebird I cannot begin to answer.
From the GM layout catalogs for Firebird and Camaro consoles.
From Firebird
97 LT1 GU2(2.73) #10297910
97 LT1 GU5(3.23) #10297909
98-99 LS1 all (both GU2 + GU5 #10279909
Hmmm, I have little reason to suspect that the previous owner would have changed driveshaft or rear ends. Mine definitely has a steel driveshaft, but matches the RPMs listed by RPM WS6 "71mph @2100rpm = 3.23s" I do not have any door or glovebox stickers, but I did not realize that I would be able to get a re-printout at a dealer. I may have to do just that.
I have seen some info in the stickies on how to ID the internals of the diff. Is there an easy way to ID the housing? Or are they the same? Actually, I have a pic of the inside from when I changed the axles. Does this help?
Housings are the same,carriers are different,there is only about 1/4" difference from the ring gear mounting surface to the center of the pinion gear between the two carriers,difficult to tell from the pic. Number of teeth(for both ring gear and pinion gear) would be stamped in both gears.
Clarification on the 1st gear lock-out. Yes,it was an LT1 'thing' and as the OPs' vehicle is a LS1 Firebird,I should not have brought it up.
BUT,the 98 LS1 Z28 had it,discontinued with the 99 + later LS1 Z28s'.
Why GM had it for 98 LS1 Z28 and not the 98 LS1 Firebird I cannot begin to answer.
From the GM layout catalogs for Firebird and Camaro consoles.
From Firebird
97 LT1 GU2(2.73) #10297910
97 LT1 GU5(3.23) #10297909
98-99 LS1 all (both GU2 + GU5 #10279909
It seems there is an error in the catalog then, which wouldn't be a first. My '98 (March build date) is an original GU2 car and it did not come from the factory with any lockout. The console is the same as a GU5 car with full access and markings for manual 1st gear.
Originally Posted by eb110americana
Hmmm, I have little reason to suspect that the previous owner would have changed driveshaft or rear ends. Mine definitely has a steel driveshaft, but matches the RPMs listed by RPM WS6 "71mph @2100rpm = 3.23s" I do not have any door or glovebox stickers, but I did not realize that I would be able to get a re-printout at a dealer. I may have to do just that.
Some of the aluminum shafts were known to develop a "tink" sound during shifts or when putting the car in gear (from park.) For this reason, as well as strength, some folks have swapped to steel shafts in place of the stock aluminum ones. Yes, as mentioned above you can get a printout of the RPOs from a dealer parts counter, but that will only tell you what was stock (this will be on a standard sheet of paper, not a sticker like what was originally on your door jamb - but the codes will be the same.) If you use the tire/driveshaft rotation method, it will remove all doubt about the current ratio.
Thanks for your assistance. I will try the wheel turning method this weekend. The axle is hanging below the car, sitting on the wheels, and no driveshaft right now. Should I lift one side only? Both sides? Should I turn from the pinion input (if I can get enough leverage), or one of the wheels?
Also, in regards to the 1st gear lockout, yet another serendipitous photo I snapped just before yanking the entire console and driveline out of the car:
I am switching to a manual, so I'm trying to decide to stick with this rear end for a while, which I can do with 3.23s, as that is actually perfect for what I want, or if I have to swap out the 2.73s, which would be too tall for a T56 Magnum. Since it is a street car, I'll opt for the wider ratio T56, so that means I won't need a crazy short rear end to get off the line, and can still take long road trips with decent highway economy and noise. The car will have around 500 hp though, and the axle tubes have a fair amount of rust on them, so I am hoping I don't break anything. If it does though, then I am simply back to buying a new rear end. Before I do that though, I'd like to see if 3.23s are the right choice for my setup by driving around this way for a bit. I also have to change the pinion seal and probably do the bearings too while I am in there, as I discovered that it is leaking.
Thanks for your assistance. I will try the wheel turning method this weekend. The axle is hanging below the car, sitting on the wheels, and no driveshaft right now. Should I lift one side only? Both sides? Should I turn from the pinion input (if I can get enough leverage), or one of the wheels?
You'd want to have both tires off the ground, and turn them one full revolution (assuming both tires turn together in the same direction - which they should if you still have the original '98 Auburn rear) while counting turns of the driveshaft. This will be harder without a driveshaft to mark. If it's 3-1/4 turns, then you know it's 3.23s.
You'd want to have both tires off the ground, and turn them one full revolution (assuming both tires turn together in the same direction - which they should if you still have the original '98 Auburn rear) while counting turns of the driveshaft. This will be harder without a driveshaft to mark. If it's 3-1/4 turns, then you know it's 3.23s.
Welp, it would appear that I have 2.73s in there. Both wheels turned together, as you predicted, and ~2.75 rotations of the U-joint for one axle rotation. Plus the steel driveshaft supports that conclusion. Looks like I will be replacing the rear end.
Any suggestions on what/where to look for a new assembly? The car will have ~500 hp, but will not be drag raced, so I assume I don't need to go bigger than stock. (Car will be a daily and a canyon carver.)
I would like to keep all of the brake lines (might actually have to replace, as some are pretty rusty at the junctions), parking brake hardware, 4-channel traction control sensors, as I am assuming most of this hardware is not included with a new axle. If I can get it custom narrowed by 0.75"/side, then I will be able to widen the rear wheels to 10.75" for 305 tires (unfortunately there are no 315 tires in the right 19" size).
Ironically, the gears are in good shape, despite the mileage, and the rear end has new axle shafts and outer bearings/seals. It is the housing and axle tubes that have rust and grit that I would like to get rid of. How much should I expect to pay for a full rear end with gears?
Sorry for all of the questions, but will I need an adjustable torque arm or control arms to set pinion angle? This will be mated to a Magnum T56 with an F-body tailshaft, so I will already have to get a custom driveshaft due to the change in trans length.
Last edited by eb110americana; 02-06-2017 at 10:16 PM.
01-31-2017, 12:55 AM
Easy Question for Auto Cars... 
