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Discussion Starter · #1 ·
Good Day All,
As I am working through some projects on my 1996 Roadmaster Wagon, I find myself continually adding to the list of things I can do. The more I remove for one project, the easier it becomes to do another. Now I am debating about replacing the rear end gears. Currently have an open 2.56 rear end. I would like to go to a Posi/Limited Slip w/3.08 or 3.23 gears.

Looking at the different types of Posi out there, the Detroit Truetrac from Eaton seems to be the best since they don't use the clutch plates. It does not appear that Eaton makes one for the 8.5" rear end w/30 Spline. This is the closest I can find. Summit lists it as being compatible, but Eaton does not. Can anyone point me in the direction for Detroit Trutrac that will work with the rear end and axels found in our cars? Also, suggestions for the best place to source the new gear sets?
197808
 

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That is it.
The discrepancy is the 8.5 vs 8.6
The later 8.6 uses a larger side bearing than the 8.5 although the gears are really the same.
When GM went from 28 to 30 spline, the stub of the auburn diff got a little thin.
The aftermarket uses the large stub .
Special bearing which is basically 8.6 inner spec , 8.5 outer spec.
All of us with Eaton diffs, clutch or otherwise have to use the " special " bearing

if you search the 913A481 on this sight, you will see countless threads referring to it
 

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… debating about replacing the rear end gears. Currently have an open 2.56 rear end. I would like to go to a Limited Slip w / 3.08 or 3.23 …
After having owned 2 9C1-LT1s with 3.08 & a Fleetwood with 2.93, I owned a 2.56 wagon.
Best thing I ever did for that wagon - even better than reprogramming - was 3.42.
(Yes, gears and programming should go together; please understand my point.)
To this day, I regret not giving the wagon 3.42 sooner. I also regret failing to give the other cars I owned 3.42.

Between my personal experience, and the ISSF's collective experiences, I implore you, consider the following …
 

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The 3.73 vs 3.42 Questionnaire
1. This is my toy, not my daily driver?
Y=3.73
N=3.42

2. I'm gonna race, or 'play hard' [for money]?
Y=3.73
N=3.42
(also see ?#6, unless you are kidding yourself)

3. I'm gonna tow/haul, or 'work hard' [for money]?
Y=3.73
N=3.42

4. I care more about city MpG, less about highway MpG?
Y=3.73
N=3.42

5. Many or mostly steep &/or very long hills where I usually drive?
Y=3.73
N=3.42

6. I've an extra $600 lying around, just in case I NEED to buy and install a new driveshaft?
Y=3.73
N=3.42

7. My valvetrain has [AT LEAST] 1.6 roller rockers AND upgraded valvesprings to support a WOT UpShift of at least 6,000RpM or higher (I've also headers)?
Y=3.73
N=3.42
(see ?#1 & ?#2)

If 'N'= 7, you tend to get >21MpG, prefer 3.08 / 2.93 / 2.563, will probably never ask this question, and yet, still uninterested in open loop tuning for more MpGs for some strange reason …
If 'N'= 4 or 5 or 6 then get 3.42, keep the 9C1/WX3 driveshaft, save on highway MpGs, and annoy cars with lesser gears …
If 'Y'= 4 or 5 or 6 then get 3.73 - and maybe a driveshaft upgrade? Or, Maybe not? Who knows? Test to find out - then ask those with 4.11 about your next upgrades …
If 'Y'= 7 then you NEED 4.11 or more, a definite driveshaft upgrade, a torque converter upgrade, headers, heads & cam … or a Gen3 / Gen4 V8 swap … did you marry well?

Upgrade the limited slip differential. If you don't have one, then you definitely need one.
If you upgrade your torque converter, you must upgrade your external transmission cooler.
 
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Discussion Starter · #5 ·
Thanks for the quick replies tonight.
@95wagon - thanks for confirming that is indeed the correct part and sharing to search by the item number. I did a quick search by that number and as you noted found many threads in relation to it. I will do some more digging.

@Marky Dissod - thanks for the detailed replies. For our family this is one of a few "fun" cars that we have. It is currently getting a new suspension, replacing all of the steering components, new wheels and tires, new stereo system with back up camera. Debating about the rear end and possibly doing some exhaust work...there are some leaks. This car will see some car group cruises and be our family road/longer trip car. May do a local auto cross just for fun and make take it up to a road coarse nearby just to see what it will do. Do not plan on drag racing or any competitive racing. Just looking to have something that rides nice, handles decent and can surprise a few people. My really fun, sleeper car (for those that don't know enough about cars) is my CTS-V Wagon, so if I really want to hammer down on something I will pull that out. That's why I was thinking 3.08 or 3.23 so that I can get a little more quickness than the 2.56 without having to listen to the car running high RPMs on our road trips. Any thoughts/feedback is greatly appreciated, now that I provide a little more information as to our intentions with the car.
 

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With the typical stock rad tank cooler feeding the stock front air to oil cooler.
I used first a vigilante 2600 , then Edge 3000+
I don't know if I ever saw more than 200 F trans temp.
With intelligent lock up settings, and PWM disabled,,they really dont seem to make much heat.
 

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Dandy,
try this, drive the car on the highway in drive.
2.56 in 3rd is almost identical to 3.73 in 4th.
3.71 ISH ( Marky resist the urge if you can)

This will give you a real world feel of what the car would be like on the freeway with 3.73
 

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Discussion Starter · #8 ·
Dandy,
try this, drive the car on the highway in drive.
2.56 in 3rd is almost identical to 3.73 in 4th.
3.71 ISH ( Marky resist the urge if you can)

This will give you a real world feel of what the car would be like on the freeway with 3.73
Thanks for the tip. I will have to wait a little while then, seeing as the car is stuck on the lift until new suspension parts arrive. I thought about doing this while I am waiting. I am going to be replacing the rear upper control arms with adjustable while I install the coil-overs so thought it might be a good time to drop the rear end out. It's only a few more bolts again later and if they are newer, it won't be that difficult. At least this gives me more to think about. And yes, I can clearly see where Marky stands :)
 

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Dandy,
May I guess that your answers to the questionnaire (guessing N=5 or 6?) point toward 3.42?
To protect the 4L60E, more gear is always better, so I'll nudge toward 3.42.
Again, BEST thing EVER for my wagon was 3.42. Wished I'd 3.42'd all my previous B-cars.
My MpGs improved with 3.42 because I spent so much less time leadfooting 3.42, as I did 2.56.
(My previous 9C1-LT1 had 3.73. While accelerating, I preferred 3.73. While cruising, I preferred 3.42.)

Since I don't know your highway cruising speed, I'll just guess 2250RpM?
(EGR stops EGRing @ 2400RpM)

2.56 & 235/70R15 in 3rd @ 2250RpM = 73MpH
3.73 & 235/70R15 in 4th @ 2250RpM = 72MpH
2.56's 3rd gear is pretty damn close to yet ever so slightly less gear than 3.73's 4th.
(3.73 * 0.7 = 2.61 … vs 2.56 * 1)

3.42 & 235/70R15 in 4th @ 2250RpM = 78MpH
3.23 & 235/70R15 in 4th @ 2250RpM = 83MpH

MOTOR - A Rear Axle Calculator

(Also, I'm of the considered and experienced opinion that the vast majority of 235/75R15 tire options outperform the ever-dwindling 235/70R15 tire options. I mention this because you use your wagon for family road trips.)

030521 645p EDIT: the above example uses 235/70R15 because the vast majority of wagons, including the ones that frequent the ISSF, are still for whatever reason using 235/70R15 instead of 235/75R15.
Also the initial and major point of this particular post is to endorse 3.42.
Increasing both the size (length) of your contact patch and improving your tire's rubber quality was secondary / parenthetical.
 
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I don't know what the price point differences are, but it's worth saying that the clutch-type limited slip differentials (NOT the factory Auburn cone-type) can be re-shimmed at least 4 times before the clutches have to be replaced. I got 40k hard miles out of the first one before the clutches needed a re-shim, and shim kits are cheap. I have not driven a gear-type limited slip rear-end so I can't speak to that. Just a thought.

On the road trips; Marky Dissod's recommended tall tires will definitely give you a little more gear margin. I'm driving around a stock '96 Impala with factory 3.08s and stock size 255/50R17 tires and it's a nice RPM on the highway. That said, with my old 9C1 when I put 3.73s on it, I only lost 1-2mpg on the highway and as long as you have resonators just ahead of the tailpipes, exhaust noise shouldn't be a problem. Personally I like lower highway rpms and am willing to sacrifice some off-the-line acceleration to get it. Or you can go with a different torque converter and offset that. I put a slightly looser converter than stock in it, so that's a variable as well.

Your other limiting factor may be finding a 3.23 ABS reluctor. They were factory installed in many of the '91-93 GM B-body wagons; even finding new gears now may be tough.
 

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Discussion Starter · #12 ·
Dandy,
May I guess that your answers to the questionnaire (guessing N=5 or 6?) point toward 3.42?
To protect the 4L60E, more gear is always better, so I'll nudge toward 3.42.
Again, BEST thing EVER for my wagon was 3.42. Wished I'd 3.42'd all my previous B-cars.
My MpGs improved with 3.42 because I spent so much less time leadfooting 3.42, as I did 2.56.
(My previous 9C1-LT1 had 3.73. While accelerating, I preferred 3.73. While cruising, I preferred 3.42.)

Since I don't know your highway cruising speed, I'll just guess 2250RpM?
(EGR stops EGRing @ 2400RpM)

2.56 & 235/70R15 in 3rd @ 2250RpM = 73MpH
3.73 & 235/70R15 in 4th @ 2250RpM = 72MpH
2.56's 3rd gear is pretty damn close to yet ever so slightly less gear than 3.73's 4th.
(3.73 * 0.7 = 2.61 … vs 2.56 * 1)

3.42 & 235/70R15 in 4th @ 2250RpM = 78MpH
3.23 & 235/70R15 in 4th @ 2250RpM = 83MpH

(MOTOR - A Rear Axle Calculator)

(Also, I'm of the considered and experienced opinion that the vast majority of 235/75R15 tire options outperform the ever-dwindling 235/70R15 tire options. I mention this because you use your wagon for family road trips.)
@Marky Dissod , thanks again for the detailed explination. I appreciate you putting some math and numbers down for me, this is much more valuable to the way my mind works over a bunch of peoples opinions. I like numbers, they are universal and are not allowed to have opinions. The tires you listed, the 235/70R15 (28") is what I am currently running and intend to go to 245/55R18 (28.6"). I wanted to minimize some sidewall for appearance but leave more than 20s to keep some for ride quality. My daily driver has 20" but I put 18" winters on and there is a huge difference in ride, but the 20s do handle a bit nicer. I figured that 18s would be a good compromise on this car. I think you have me sold on the 3.42 gearing. I usually run about 5 mph over posted speed limits which puts me at 75 on the interstate in most states. Again, thanks for providing some data for me to review.

I don't know what the price point differences are, but it's worth saying that the clutch-type limited slip differentials (NOT the factory Auburn cone-type) can be re-shimmed at least 4 times before the clutches have to be replaced. I got 40k hard miles out of the first one before the clutches needed a re-shim, and shim kits are cheap. I have not driven a gear-type limited slip rear-end so I can't speak to that. Just a thought.

On the road trips; Marky Dissod's recommended tall tires will definitely give you a little more gear margin. I'm driving around a stock '96 Impala with factory 3.08s and stock size 255/50R17 tires and it's a nice RPM on the highway. That said, with my old 9C1 when I put 3.73s on it, I only lost 1-2mpg on the highway and as long as you have resonators just ahead of the tailpipes, exhaust noise shouldn't be a problem. Personally I like lower highway rpms and am willing to sacrifice some off-the-line acceleration to get it. Or you can go with a different torque converter and offset that. I put a slightly looser converter than stock in it, so that's a variable as well.

Your other limiting factor may be finding a 3.23 ABS reluctor. They were factory installed in many of the '91-93 GM B-body wagons; even finding new gears now may be tough.
@sherlock9c1 thanks for additional information. I am sure the clutch-type work fine, but I have this odd mental disorder you can ask my wife about. I have a really hard time not doing things the best I can. Although we won't be putting many miles on this car, in my mind the gear-type LSD units just appear to be a better design and will require less potential maintenance. Since I now know they exist, if I use what I feel to be a lesser part, it will just eat at me.

She hates when I tear into projects. Here is how things have gone on the Roady so far. Original intention was to put in lowering springs and new shocks and a stereo upgrade. It has now turned into:
-Couldn't find any solid info on springs/shocks, it is all opinion based so I am going with adjustable coil overs so I can dial in what I want.
-After disassembling the suspension, I looked at the old front control arms and figure it would be a good time to replace them...tubular control arms it is
-After realizing that the ball joints were original, I figured all the other steering components probably are...removed and replacing all steering components
-Coolant reservoir is leaking - replace. Looked at the coolant hoses, aged, figure I might as replace them as well
-Trans cooler lines are leaking - remove and replace
-Appear to have a leaking power steering pump. Will probably send that and the steering box into Lee for a rebuild and ratio upgrade. Since everything is out it is easier to get to
-This leads to debate about replacing the steering shaft or just installing an HD rag joint
-Upgrading the battery cables with a Innovative Wiring Kit
-Rewiring the electric water pump with an Innovative Wiring Kit as the previous owners wiring is a rats nest
-Stereo upgrade with back up camera (this is easy)
-Have a rear main seal leaking, and a hard park to drive shifting on the trans. Might just pull the motor and trans to make inspection and replacement of seal easy
-If I do, this may lead to headers and a trans rebuild
-Resonators have been removed from exhaust and there are some leaks and a drone at hwy cruising speeds. Trying to figure how to address this
-The rear end rebuild comes from since I will have the suspension removed, and intend to remove the lower rear control arms to clean, and clear coat (Metco Aluminum Arms) and will replace the uppers with adjustable control arms dropping the axel to rebuild it out of the car and then take it to my mechanic for final adjustment inspection would be easy
-Since the spindles are out, good time for a front brake upgrade.

Again, when I find things that are off, I cannot help myself to fix them which leads to the "while I am this far" syndrome. Hoping to have most of this done by May...we will have to see just how far I can get. I am hoping I am not the only one here that suffers from this type of mental disorder.
 

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Discussion Starter · #15 ·
Ahh, the station wagon of my dreams...
Mine too, mine too. I fell in love with them when I watched Hit & Run with Dax Shepard back in about 2014. I had no idea what wagon was keeping up with the Corvette and had to figure out if it was movie BS or a custom build. To my surprise it was something that could be purchased. I don't know what I did to deserve her, but this Fall my wife told me...you really should start looking for a CTS-V Wagon, so I did. Found this beautiful 2013 with 4300 miles on it. It was delivered the beginning of December, it has been a long winter looking at it sitting in my shop. As soon as the snow and salt have been washed off the road, she will be taken out for a rip!

197815
 

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Too many people make too many decisions about these cars with no objective data.
This forum is supposed to distinguish itself from lesser forums by the quality of its info, some of which should be objective.

To that effect: not 235/70R15 - I stopped recommending this tire size many years ago. Let me be clear:
235/75R15​
IF I'm lucky enough to own another Caprice RoadMaster or Fleetwood, I WILL NOT settle for a tire shorter than 28.9" tall.
The very mild RpM drop is a very mild bonus.
The best tire choice in any other given 'OEM' tire size is inferior to a midpack 235/75R15 tire choice.

In your case I'd use 235/60R18, or 255/55R18 if the wheel width dictates a wider tire.

Most performance mods are maintenance upgrades.

In my opinion, 2.56, 2.93, 3.08, and even 3.23 are not enough axle gear for LT1 B- & D-cars.
I also believe that the CTS-V wagon was another GM blessing cursed to be short-lived.
 
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Discussion Starter · #18 ·
Well Pirelli ( my go to ) makes a 255-50-18 Pzero ,,,,
Kind of a balance between tiny doorknob short and circus clown tall
The 255/50-18 only seems to have the Pirelli available and I am not into spending well over $400 per tire. In addition, I may not have enough width on the back. With the wheels I am putting on, I will need to add a 1/4" spacer for the brake kit on the front to clear so I will want to add that to the rear as well so the rear track width isn't less than the front.

Too many people make too many decisions about these cars with no objective data.
This forum is supposed to distinguish itself from lesser forums by the quality of its info, some of which should be objective.

To that effect: not 235/70R15 - I stopped recommending this tire size many years ago. Let me be clear:
235/75R15​
IF I'm lucky enough to own another Caprice RoadMaster or Fleetwood, I WILL NOT settle for a tire shorter than 28.9" tall.
The very mild RpM drop is a very mild bonus.
The best tire choice in any other given 'OEM' tire size is inferior to a midpack 235/75R15 tire choice.

In your case I'd use 235/60R18, or 255/55R18 if the wheel width dictates a wider tire.

Most performance mods are maintenance upgrades.

In my opinion, 2.56, 2.93, 3.08, and even 3.23 are not enough axle gear for LT1 B- & D-cars.
I also believe that the CTS-V wagon was another GM blessing cursed to be short-lived.
I understand your recommendation for a 235/75R15, however in your RPM example it was given with a 235/70R15, which is what is currently on the car.
 

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Before I go,,,
You realize the wagon is already a triangle with its wider than sedan diff ?

Track Width, Front (in)62.1
Track Width, Rear (in)64.1

My current rear disc setup moves my current diff track out by almost .25 per side with 255s and is OK
If you are planning , as in not bought yet, different front brakes that require spacers, maybe look at Tobin at Kore3 or Tod at TCEs offerings.

Tires are a giant " you get what you pay for" as you all ready know
I assume your V is not riding on Chaoyang tires ;)

Anyway, good luck with your project, looking forward to seeing the outcome.
 

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The 255/50R18 only seems to have the Pirelli available and I am not into spending well over $400 per tire.
For 235/60R18
Pirelli Scorpion Verde AllSeason Plus II 740AA 107V $173
Continental CrossContact LX25 740AA 107V $178
FireStone Destination LE3 700AB 103H $163
Not saying you would drive these in rain / snow, but their rain / snow ratings are better than good enough for your family.

For 255/55R18
Pirelli Scorpion Verde AllSeason Plus II 740AA 109V $194
Continental CrossContact LX25 740AA 109H $200
BFG Advantage T/A Sport LT 660BA 109V $189
FireStone Destination LE3 700AB 109H $186
Vredestein QuaTrac Pro 400AA 109W $180
In addition, I may not have enough width on the back. With the wheels I am putting on, I will need to add a 1/4" spacer for the brake kit on the front to clear so I will want to add that to the rear as well so the rear track width isn't less than the front.
Per 95wagon, a wagon's OEM rear axle trackwidth is 2.00" wider than the front, so you will NOT need rear spacers.

From my wagon experience:
1. A 255mm wide tire will not fit into the wagon's spare tire well regardless of wheel size.
If it somehow does, you risk hurting yourself removing it.

2. Back when my wagon wore 255/65R15 (well before I reprogrammed it for 235/75R15), my rear outer sidewalls were worn down to anonymity over the course of several months by my funloving cornering antics.
To reiterate: you will NOT need rear spacers.
To add: maybe massage the parts of the rear wheelwells that threaten the rear outer sidewalls with 255m wide tires.
 
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