IIRC the 4th gen rears are something like 2-4" wider than 3rd gen stuff. I have a 3.27 with posi removed from an 88 GTA. I like the extra strength and the bearing retainers rather than the c-clip. IIRC they came in 2.73 as well and there's 3.70(3) aftermarket available. Any others? No abs in 3rd gens right? I'm guessing 93 was the first year for that? Big deal to swap in a 9 bolt, what maybe some wheel spacers, longer studs, different wheels? Late 9 bolts were set up for the PBR calipers right, 89 and later? all from memory. It's either this or swap in a 12 or 9, which I did with the GTA, or swap carriers and ring and pinion for a 2.73/2.56/2.41 in the 7.5? Anyone have an 89-92 around with 2.73s in it and discs? I'm in the greater Los Angeles area?

Not sure of all of your questions, but...

1) 4th gen rears are 10 bolt 7.675 rears - I don't think there were any 9bolts in the 4th gens - the LT1 rears (93-97) are almost 4 inches wider (almost 2 inches each side), the LS1 rears (98-02) are slightly less from what I read - I have a LS1 rear and it only sticks out about 1.5 inches farther each side. LS1 rears have much better (larger) discs, so a 16" wheel is needed (as well as a 16" spare - everyone forgets about the spare)

2) No ABS on any 3rd gen rear

3) a 9bolt from a 3rd gen will bolt right in, no alterations required, and no wheel fitment issues, as it's the same as a 10bolt length wise.

You're sort of right. Keep in mind the last year for the 9-bolt was 1989, so there's only one year that fits your requirement (9 bolt and PBR discs). Also the 9-bolt ratio is 2.77, not 2.73. There was also a 3.08, 3.27, 3.45, and 3.70 sets from the factory. Any benefit of the BW 9 bolt is very debatable.

That's right is 3.27 vs 3.23 and 2.77 vs, 2.73

I may end up with a 95 A4 Z28. It probably has a 3.23 in it. Based on my experience with a factory 3.27 behind a 700r4 (same ratios as 4L60 etc.) it's too low down low and still too low in 4th. Economy suffers, noisier etc. I think the highway mileage figure on my car was 19 with the 700. I swapped in a T56 and used a 3.70 Ford 9 rear and hit that nice 1500 rpm @ 60 mph give or take. WIth tuning etc. I once turned in over 30 mpg with the T56. lean burn advanced timing , carefull accelerator movement....60 mph.

I notice factory 2.4x, 2.56, 2.73 ring and pinions are still available but appear to be pricey. The 7.5(625) rears do a better job than I'd expect, but I still don't like the c-clip deal as many race sanctioners don't allow, which is part of the reason I switched to a 9". However, the 9 bolt, though somewhat rare, gets rid of the c-clip and offers at least one ratio I'm interested in behind an A4...2.77 for my intended use,,highway trips, some long, not just local cruising or stop light to stop light where the 3.23 would probably get the nod.

If I pick this car up and drive it to SoCal from Florida, the whole 2000+ at 70-75 mph is going to start bugging me right off the bat. I think the car would do fine with a 2.56 or 2.73(7) rear. My guess is mileage would pick up 2-3 mg. I've considered re-installing my 3.27 9 bolt in place of the 3.70 9" just to see what happens....

also swapping in a used rear with the right gear and diff, should be less expensive and simpler than purchasing a new carrier, ring and pinion, etc. installing it, having it installed. Pickihg up or trading a used 3.23 for a 2.73 if keeping the 7.625 might make the most sense.

It shouldn't be hard to find a free 2.73 rear end. Having had 2.73, 2.77, and 3.08 rears in a couple thirdgens, and 3.23, 3.27, and 3.42 gears in other thirdgens, I would never consider spending money on 2.77/2.73 gears. I've always gotten fair mixed mileage with 3.23s, and on the highway with the 700R4 in overdrive the stock L98 doesn't even begin to break a sweat. I don't forsee a significant real world different in interstate gas mileage between 2.7x and 3.2x. It's just not a huge difference when in OD.

I hear you and will tend to defer to your greater fbody experience than mine, however, that's an 18% difference in gearing. SOme try to claim an equivalent increase/decrease in economy, which those of us who know anything, know doesn't materialize. However, a 10% inprovement in cruising economy wouldn't surprise me, which is what 2 mpg or so, maybe 3. I'll see what happens if this deal comes together. Got other non-Fbody stuff I'm looking at as well.

Anyone reading this want to consider a straight trade for a 2.73 geared 7.625 for a 3.23, let me know. I'd be looing for posi. Also, I'd consider trading a 3.27 geared non-PBR caliper '88' rear for a 2.77 preferably PBR caliper rear. If it comes together I'll put want ad out.

Thanks for your input.

thats an 18% difference, but the OD tends to make the difference less.
you have a 0.7 OD, so your rpm difference between the two is only like 150-200 RPM i think.

whatever it is, take the percent difference between the rpms and THAT will tell you your gas savings.

I have been DIEING the see a 3rd gen rear (or at least the same width) in a 4th gen! Imagine the deep dishes of the wheels you could run in there!

you mean like thirdgen wheels?
a 4th gen is no wider than a thirdgen.
the bolt pattern is the same.
you can run deep dish wheels in a 3rd gen with our stock axles.
add to that that 4th gen camaros are plain ugly anyway aside from the late LS engined ones

The width difference is in the axle itself. Nobody ever said the difference was in the body.

I was referring to how 4th gen axles prevent running deep dished wheels, on a 4th gen. With the narrower axle, they could run a deep dish.

to each their own. I think they all look good, just in different ways, with the early 4th gens being the 'swoopiest' most batmobile looking.

the one i'm looking at has 18 inch ZR1 wheels on it apparently. I like the sound of the deep dish rear concept. I'll keep it in mind.

what i was saying is that in stock form, we dont have deep dish wheels at all.
we have the same body width, so in theory deep dish wheels should fit on our cars... yet you dont see any.
just an observation i guess