Allmyin

I have a 1970 cutlass S. It was a 350 2bl car with 2.56 posi rear th350 trans. About 2 years ago I installed a 455 , .40 over full roller..
I would like to change the rear gears to make the car more fun to drive. I do city and highway driving. I am planning to switch to a OD transmission 200R4 at some point. Sooner than later. I do not race. The rear now is a 12 bolt. That’s about all I know about it. I’m sure it the one with the 10 bolt guts in it or something. Not sure. I do run 295/55/15 on the rear. At 65 mph my rpm’s are 2100 or so. The car cruises at 70 mph all day with no problem. What gears fit in the carrier, I already have? Any help would be appreciated

Fun71

Unfortunately there are no other gears that fit the 2.56 carrier. Most of the aftermarket gears are made to fit the 3.08-3.23 series carrier. You will need a different carrier for any of the 3.xx ratios. There are new carriers that are Ford 8.8 units machined to fit the O-Type gears and bearings. Also be aware the ratios available for the O-Type are very limited.

Factory: 3.08, 3.23, 3.42, 3.91, 4.33, 4.66, 5.00
3.42- up use a different carrier than the 3.08-3.23.

Aftermarket: 3.42, 3.90, and 4.10. All fit the 3.08-3.23 carrier. If you can find them.

joe_padavano

Just so you know what you have, that axle is the Type O axle, only used in A-body Oldsmobiles from late-1966 through the end of the 1970 model year. Aftermarket parts are limited at best. As noted, you have a 2-series carrier that only accepts 2.56 or 2.73 gears. It IS possible to use a spacer behind the 3.08 or 3.23 gears in that carrier, but it would have to be custom-machined. And yes, despite the twelve bolts on the cover, this axle uses an 8.5" ring gear with ten bolts. Nothing is common with the Chevy ten bolt axles.

Allmyin

Thank you so much your information. I appreciate it. So my only option is to change out the rear if I want a different gear? That sucks.

VC455

Here's one man's opinion...

Your rear tires are very tall with a massive tread. With a 200-R4, a 3.90 rear gear would give you an overall ratio in first gear of 10.69:1 (plus converter multiplication). You would have a very responsive feel, even with those big tires. That's a deep enough gear that a standard torque converter would work OK to preserve your fuel economy.

Your effective ratio in top gear would be 2.61--so you'd see virtually no change (+2%) in highway rpm from your current 2.56.

VC455

Fun71 said: "There are new carriers that are Ford 8.8 units machined to fit the O-Type gears and bearings."

You need a new carrier and a new ring and pinion. You do not need to change out the rear.

Fun71

This from another thread may be helpful:

https://classicoldsmobile.com/forums...ebuild-177376/

GEARMAN69

The oem 2.78 fits your carrier , just find a 67-70 Cutlass open rear on the cheap and grab those gears.. Not sure on Type O spacer availability to put a 3.08/3.23 gear on yours .
I run a 295/55R15 M/T DR's on my sons 74 C10 454 and it has 3.07 gears and cruise 79 mph on interstate is right at 3000 RPM.

BangScreech4-4-2

I question whether it would be worth the effort to switch from 2.56 to 2.73.

Fun71

Agreed. Back in the 80s I switched from 2.56 to 3.08 and it was noticeable, but 2.78 wouldn’t really be worth the effort. With an OD transmission I would do 3.90 or 4.10 as said above.

GEARMAN69

I am just answering his question.. He says he doesn't race and also talking about an 200-4R OD later when it totally does not need one currently. If it was me I would run the 2.78 AND run a shorter tire and keep the three auto.

olds 307 and 403

You don't need a whole different rear but the aftermarket 3.42 or 3.90 and 8.8 modified posi isn't cheap. Do a gear swap that at the same time as a built 2004R. I picked nice used 3.08 gears and new 8.8 posi. I plan on installing them in late fall.

Koda

Can you clarify what you mean by more fun to drive? Leaving the line?

GEARMAN69

For the non drag racing folks also a strong 455 does not need 3 series gear to be quick off the line , just have a dual plane intake not a single plane and not be way over cammed and has some decent compression especially.
MANY 12 sec BOP 455 cars out there doing it or have done 12's on the track with a 2 series ratio... Hell my buddies heavy AF 66 Catalina drive anywhere 93 octane 455 iron heads th400 2,93 gear at a 4500# RW +/- runs a 12.8 and 8.22 at the 1/8
I did 12.6 in my old 68 GTO with 2.93 , there is the guy with the stock appearing full wheel cover 70 Monte Carlo with 2.56 gears and a oval port 511 cid runs 11's on Cooper Cobras driving hours to the track.

There is zero reason to spend a ton of money to convert to a OD trans that was meant for under 300 ft lbs and change to a low gear set (Small block mentality) when a proper 455+ CID torque monster can drive as designed super star with 2.56-2.78-3.08 gear what ever and a three speed auto.. This guy is running an a big tire with a tall 2.56 combo is NOT ideal but if it was me I would just run a shorter tire and leave it be ... if it had 3.08s , that tire is a better match and keep the 3 speed .

My sig , sons truck I race has same tire and 3.07 gear 4400# and no slouch with a Torque monster 9:1 std bore mild 454 (zz502 hyd roller cam 224/234 110) drives where ever it needs to go and no slouch off the line 8.3-8.4 in 1/8th
1974 C10 Cheyenne Super Gen 6 454 L29 13.1@102 last year did it on a STOCK 12" converter not a 2500