Ok....I have my 455 and a TH400 for my 70 Cutlass S but if I ever want to move and not just spin one tire (just kidding of course), I'll need to replace my open 256 rear end. I'm definitely going with a posi rear but what sort of things do I need to consider in choosing what ratio to get? Also, I see there are different coded rear ends of the same ratio...what is the difference between those? Car will be a sunday driver kind of deal but I also want it to be stout. As always, thanks in advance.

 

You need a posi for sure. Anything with a 455 should have come with one from the factory! I have a hard time NOT spinning a tire around a corner with Lady's 350 and 2.73 rear...

For a Sunday driver, and not really caring about mileage, between 3.27 to 3.73 (not sure of exact Old #s) would be good compromises between fun and economical... With a 455, the 3.27 should be sufficient IMHO.
It really comes down to your driving style... Want more get-up-and-go or more mileage?

I think there is a cross over point where the carrier size changes (more $ maybe), but not sure where that is...
I will let the rear-end experts chime in here...

 

I was looking at a gear change too, and I think the cutoff is 3:42's I have 3:08's in mine, but I have also been "told" that with a 455 and a 4 speed the 3:08's might just be fine!! go figure I was thinking 3:42's had a 69 geetoh with 400 and 3.55's and it was quite spritely

I also read you want to keep a 455 under 4500 rpm...

 

My 2 cents for all around driveability the 3.23 for an automatic transmission and the 3.42 for a standard. Years ago I bought a 1968 Cutlass with a 350 and 4 speed, it had 3.08 gears. It was hard to come off the line without slipping the clutch and sometimes I would "leap" through the intersection when I really wasn't trying to. Money was tight in those days so when I came across a Chebby 3.31 rear end I installed that under it. Much nicer to drive on a daily basis, especially for stop and go in town. John

 

I agree 100%

 

x2

That should really wake your car up!

I went from a 2.56 open to a 4.10 posi. Smoked em right in the driveway after the swap and I wasn't even trying. A lot of fun but I am going to put 3.42's in for cruising. The 4.10's are fun around town but definitely not on the highway!

 

I would agree w/ the earlier posts about 3:23 for Auto and 3:42 for stick cars. Those are good "middle of the road" gears that will have enough to come off the line well, yet still can comfortably do highway duty.

I have several other cars w/ 3:55's which are also great, but at 70 mph the motor is starting to work more than I would like (IMO) for longer hauls.

Just my two cents

Either way, you'll notice a huge difference coming from your current gear!

 

Go with a 4.88 spool if you are a REAL MAN!

Really though, a Cutlass with 3.42 or 3.73 would be a good choice. For a Delta 88 or 98 I would use 3.08 or 3.23.

 

Based on input so far I had settled on looking for a 3:23 posi to put behind my 455 but now I have an opportunity to get my hands on a 3:08 posi at what I believe is a very reasonable price. If I were to drive two identical Cutlasses with 455s, one with a 3:23 and the other with 3:08, would I experience a significant difference? Any and all input is greatly appreciated.

 

Obviously the 3:08 will get you a bit better gas mileage, but probably not really enough to tell any big difference in the pocketbook.

Candidly I don't understand why anyone not heading to the strip would want anything less economical than a 3.08 and or maybe even a 2.73 behind a 455 unless the deal is to race to the nearest gas station and back. Neither ratio or one in between is going to make the car seem like a slug with typical driving.

 

With a 455? If it was my car and I was using it for highway (70mph I think you said) I would really look hard at that 3:08 posi. It will give you the bottom and top end you're probably looking for. Would you notice any difference between 3:08 and 3:42s on your car? Sure, you'd buy more gas, and you would be faster off the line, but how much? Totally up to you. I think it still comes down to how you're planning to run the car. Daily driver? Highway? Just for fun? Maybe the best compromise is go in between. Pick up your posi, sell the 3:08s or hold onto them, and swap in some 3:23s.

My car has a 350 4bbl and I'm changing it from econo gears to a posi and 3:42 gears. It would be an interesting hole shot between the two, but the 455 with the 3:08 would win hands down. It just has that much more power in the same weight car.

 

IMHO,
A 3.23, 3.31, 3.42. Are about as low numerically as you would want in a 4 speed with a wide ratio. I think an automatic would be better suited for a 3.08 or a 2.73 ratio.

 

I like 3.08s behind a 455.

 

I run 3.08's behind my 455 and turbo 400 in my 57 and can still frys the tires until I let up on the gas. makes a great daily driver

 

I run a 3.91 rear in my 70 442 with a M21 4 speed.

The good: wicked low end acceleration, great for 0-60 and street cruising

The bad: too many RPMs on the highway

Since I never go on the highway, it is no problem.

It depends on what you do with your car.

If you use it as a dialy driver, i vote for 3.08, with 3.23 as a second choice.

 

I vote (and am using) the best of both worlds 3.73 with 5 speed overdrive

 

I also like a 3.08 and it will "run like a scalded dog" especially after it's rolling. Like getting on an entrance ramp.

 

My 70 W-30 came with a non-original 3.08 anti-spin. With the stock close ratio Muncie and 27" tall 255/60-15 BFGs, it is a real dog off the line.

 

I am not a fan of the 2.20 first geared Muncies. I bet your 70 Olds will go 60 mph in first gear.
We opted for a 5 speed in our 65 Biscayne. It has a 3.27 first gear and a 1-1 5th. That combo would really wake up your Olds, It sure woke up our Chevy.

 

Anyone know the approximate weight of a rear end? I need to get a shipping quote. Is a 3:08 posi O type for a cutlass. No brake parts.

 

What can anyone tell me about this just from looking at the pics. My understanding is it is a Type O posi rear.

 

OK- well Here we go...

The Housing IS a 12 bolt Oldsmobile 'O' type rear all the way.

I would like some close up pictures of the posi unit na dthe ring gear bolt area if you can.

WHY? We it looks to be a EATON posi in that housing and EATON did not supply a posi for this 12 bolt 'O' type...So I want to see if some one modified something or what. The 'S' spring also DOES NOT belong in a Eaton posi either...This si very confusing on the POSI part of the rear end.

Please send more posi and gut pictures the beast you can.... If you can remove a ring gear bolt that would alos help and numbers from the ring gear diameter also. Jim

 

I agree. Once its rolling 25-30 mph a 3.08 is good to go. Off the line it's not the best. But it's bettter on the hwy. I guess it depends on what you are going to do and wheather or not you concern yourself with mundane issues like MPG. I have a 3.23 also. That might be a good compromise.
But, I don't race anymore.

 

Monzaz....thanks for the input. Can't provide anymore pic cuz it's not mine. Was a unit I was considering buying, well actually, I had bought and was to be shipped tomorrow. I looked at some Eaton pics and it appears you are correct. Based on this I backed out of the deal. The guy didn't have a problem with it but did state that this info answered a couple questions he had about it. Well, guess I'll keep looking. Thanks a ton for the info!!