Hey guys, I'm looking for a new rear end for my 1991 Trans Am convertible. It has the stock garbage 2.73 and I'd like to replace it with something better. It also has the 700r4 auto transmission and a 305 TPI. What do you guys think would make the best of the car? I'd like better launches but comfortable highway cruising as well. This car is gonna be close to a daily driver in the warm months. So its either 3.42 or 3.27 (or something very close). What do you think? Thanks!

 

I have the 700r4 with 3.23 (i think) and its great. spins the tires in first gear and again in second gear when i punch it. The previous owner might have done something to it though, I am not sure.

 

Thanks for the quick response! Yeah you might have a shift kit in the transmission, and you have a 355... I have the lowly 305 lol.

 

I would suggest using 3.45,3.42 just to get the car rolling easier. It would be better if you had a manual trans but it will still help.....

 

i actually havent driven it with the 355 yet, hopefully get it started for the first time tonight, the 305 had 170 k miles and still performed pretty well.

 

What can you find cheap? 3.23 vs 3.42 you're not going to feel much difference. I'd avoid 9 bolts like the plague. Nothing against them except the complete lack of aftermarket support, and the expensive PBR disc mounting brackets. Well, that and the complete lack of aftermarket support. I'd look for a 91-92 10 bolt with 3.23 or 3.42 posi and PBR discs, second choice would be a 91-92 10 bolt with 3.23 or 3.42 posi and drums, then I'd start looking at 4th gen rears, followed shortly after by rebuilding your existing rear end with a 3-series posi carrier, new gears (of your choice), and new bearings. Truth told rebuilding your existing rear end, while admittedly pricey, is probably your best bet. Because, when you're done you know it's all new. Any used rear end is probably going to probably have been abused and the stock carrier in most of these cars isn't rebuildable.

 

2.73 swap to 3.23,3.27 or 3.42 will definately feel a difference..
I swapped a 2.73 rear for a 3.23 rear the car went from 15.0 to 14.7

I encourage you to find a rear end from a TPI 305/5spd or get a rear end from a 4th gen then you get larger brakes,better gear ratio and a better LSD unit....Sacrifice is rear wheel offset,you have to run 4th gen spaced wheels out back.....

 

I've done 2.77 to 3.27 and then 3.27 to 3.42 and i felt a difference each time but surprisingly the 3.27 to 3.42 swap felt the best. Track performance did pick up with each gear as well.

I'd suggest 3.42's. Great highway cruising still with great acceleration feel.

 

If highway cruising is a requirement, lower is better and thus I say 3.2X.

My car is pretty peppy in 3rd with my 3.42's, but I can't comment otherwise as 1st was shot and it wouldn't stay in 4th on the highway.

 

When in doubt gear it out!!

I daily drive my 3.73s with a non-overdrive turbo 350. Even on the highway and interstate. I think you'll be okay with any lower gear if you have OD and a cam that likes that particular rpm range.

 

Thanks for the feedback guys!
It seems that people are leaning towards the 3.4Xs....

Well I found a 3.45 in northern mass for sale for what seems to be a decent price. I'll see if I can talk him down a bit further since the drive will be far for me. And as for aftermarket support (what Drew mentioned)... who cares? If the unit works fine, I don't plan on doing anything to it... MAYBE bigger brakes in the future but that's it. And anybody know what I should look for when opening up the diff cover? If anybody has pictures of a worn posi verses a good posi I'd love that so I know what to look for. Thanks!

And I'm considering getting a 3.2X since I see them for sale everywhere, that is if I don't find a 3.4X for a decent price. The 3.2Xs seem to almost grow on trees.

 

i really like the 3.42s in my car,highway cruise is about
2800-3000rpm,right in the thick of the torque curve of
my engine combo
I'd check that 9 bolt over real good,of the 3 i have had
every one had a badly worn posi-if there is fine metal
dust in the oil the posi is worn,will damage the bearings
if ran long enough with contaminated oil
If you find a good 9 bolt,install a drain plug in the
cover and change the oil every year or so
the 1990-'92 3rd gen rears have upgraded axles and
those 28 spline axles can be installed in earlier rears
if the matching carrier is used. 4th gen uses the same
ring +pinion and same size carrier as 3rd gen,though
a torsen carrier is used on '98-up.

 

I went from 3,43's to 3.73's. Felt a big diff! Went side ways with the 3.43's then went crazy with the 3.73's! Good highway drivability. Hell i'm gonna be selling the whole rear if you want to go a lil lower gears.

 

What cars came stock with a 9 bolt 3:27 set up. Thats what was in my 1982 trans am when I bought it.