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What differential do I have, and what type is best to have?

aroethli

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1995 RM sedan:

GM8 AXLE REAR 2.56 RATIO


Is this just a standard "open" differential, versus limited slip or posi-traction?


I've seen a lot of discussion about different gear ratios, limited slip, and posi rear ends that have been in the RM and its sister models.

For day-to-day driving, what is really the "best" rear-end to have for reliability and fuel economy? Is there a particular rear-end that maximizes both performance AND economy?

Thanks for all the help!
Al
 

Goldmaster

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Yes, you have an open differential (not limited slip). The 2.56 rear will get the best mileage if you do a lot of freeway driving. I recently installed a 9C1 (Police Package) Disc brake rear end with a 3.23 limited slip under my RM. So far it doesn't seem to have affected my mileage too bad. But I won't know for sure until I get a properly programmed PCM, as my speedometer is way off since switching from the 2.56 to 3.23 ratio...All I know is that it's got a lot more pull from a dead stop now :D If I had to guess, I bet my city driving fuel mileage may actually increase if I can keep my foot out of it. The car doesn't have to work as hard now to get moving from a dead stop....
 

aroethli

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Thanks,


For just normal driving, not racing, not pulling a trailer/towing, etc.... what is the best type of differential to have, limited-slip or posi?

I suspect I'll leave mine alone, unless it wears out, but am just curious.

Thanks,
Al
 

MrBuick (Magnum)

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a limited slip IS a "posi" tractrion. just different terms.

Limited slip
Positraction
Twin grip

All the same thing, really. There are different type of limited slips, most common is an Eaton clutch plate style, or the auburn cone clutch style. There are also mechanical lockers, electronic lockers, Worm gear styles, and spools. All for different uses. Clutch styles suit most peoples needs, and can be used on the street without fear of damage to the axles.

Best ratio for performance, while still keeping economy? I'd say 3.08, like GM used in the 9C1's, and the Impala SS. but 2.93 like they used in the Fleetwood "Broughams"(not the base models), and used in the Roadmaster/Caprices with the towing option, can still have very great milage. My dad got 23mpg from florida, to michigan in my Fleetwood Brougham, and I know he can have a bit of lead foot.

3.23 might be a little bit on the performance side of 3.08's, or 2.93'w, but still very economical. They really should have used that ratio more.
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aroethli

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Ah, that's very helpful.

I thought that might be the case, but hadn't looked into rear-ends in such a long time, I couldn't remember.

So, if my 2.56 ever blows up, can I just have the guts changed or would I need a whole donor rear end to get a 2.56 or 2.93 limited-slip?

Thanks again,
Al
 

kiltedscotsman

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Well my 95 RMW has a tow pack installed, but I have NO idea what final drive ratio it is. Im assuming 2.93, as she seems to get good milage on cruise (sometimes 28mpg). My mech says I have a limited slip diff.

Furthermore, I have no idea what a limited slip diff does...:p I just know that it recently got a rear end service and seems great! :D
 

92RMSJMM

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what is the most likely car to have the 3.08 lsd rear end i've got 2 cars near me with limited slip rear ends in them (a 94 RMS limited and a 91 9C1 caprice) i would like to put an lsd in my car before summer is over

and is it better to leave the fan clutch on my new alluminum fan or just take it off?
 

Buffman

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a rms lt1 car will only have 2.56s or 2.93s. Not sure about the 91 9c1.
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aroethli

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In non-racing/towing applications, is there really any disadvantage to having the open differential 2.56 rear-end?

I'll be doing mostly highway driving.

I don't plan on changing it regardless, unless it breaks one day, but I'm just curious about he best application.

Al
 

MrBuick (Magnum)

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one thing you need to know, is the 2.56 gears, use the 2 series carrier, which means uses gear ratio's of 2.56-say 2.00. a 3 series carrier uses gear ratios of 2.73-4.56. you can put 3 series gears on a 2 series carrier, with a spacer, but no the other way around. so if the posi is from anything but a 2.56 car, the gears need to be swapped as well. the 94 RMS with the posi, could be either 2.56, or 2.93... but a 2.93 with a posi is more liekly, since the 2.56 with posi seams to be a no so common option. I know the 9C1's tended to use the 8.5" rear, but I donno about the 91-93 cars. Most caprices can stock, with the 7.5" rear... which is something you don't want... they are WEAK. It would also prolly be cheaper to swap the entire rear, into your car, vs just swapping the posi, or gears and posi...
 

92RMSJMM

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ok thanks for the help i had planned on swapping the whole rear because i've read your guys' past posts and it sounded like that was the best way to go
 

aroethli

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I'd love to have a LSD "just because" :cool: ....but I'll leave my open 2.56 back there until it dies I suspect. It does deliver good highway MPG, which is my goal anyway, so it probably is a good solution.

Al
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MrBuick (Magnum)

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I'd love to have a LSD "just because" :cool: ....but I'll leave my open 2.56 back there until it dies I suspect. It does deliver good highway MPG, which is my goal anyway, so it probably is a good solution.

Al
An "open" diff is pretty hard to "kill", since it's just spider gears free wheeling, and the gear sets can last more than 500,000 miles if the oil is changed...
 

Ohio_tech

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I have a 95 RMS with G80 Posi, GM8 2.56 gear ratio. Only 72k miles and after a while it starts popping/catching on the limited slip after 30 to 45 minutes of freeway time when going slow as in stop-and-go traffic (3 to ~12 MPH). I want to change the fluid, but have never taken off the rear plate to see what type of limited slip it is. Given what I have told you already, can you please tell me if it is clutch type, cone type, or other? As a retired auto-tech from a previous career, I thought clutch type LSD take a different additive and weight of gear oil than cone type.
 
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