Hello Experts,
Looking for a solution. I have a 70W30 with the original 12 bolt posi 3.91 gears. I want to gear it down ( excuse me if it’s not the right term).
Want to lower the gears in an effort to preserve the original engine so it is not reving so high on the freeway @ 60 mph.
I was told 3.08’s would be a good ratio for highway driving & keeping engine RPM’s low?
Questions:
Is the 3.08 right gears? Where would you purchase them from ( Supercars does not carry).
Can I use the existing rear end?
One possibly swap out the whole rear end with a another Olds 12 bolt posi from 67-70 cutlass/ vista cruiser?
I’m open to all suggestions, & I know what you Olds purist are saying just leave it alone & drive slower.
Thxs, ABF

 

Your carlikely has an o type rear end. Gears are not so commonly availible for that rear end. Its not uncommon to find some good used original gears toreplace what you have.

a later 71-72 cutlass 10 bolt rear end will bolt right in place of your O type rear end and has many gear and posi options availible for it.

i reccomend sending user monzaz on this site a pm. He owns a shop and is well versed in olds rears.

 

The Olds O-Type rear had a limited number of gear ratios, and used a different series carrier for the different gear ratio ranges.

Here is a list of the posi units & what gears they will fit:

#673 = 2.56 & 2.78 ratios, 28 spline
#671 = 3.08 & 3.23 ratios, 28 spline
#588 = 3.42 & anything numerically higher, 28 spline
#672 = 3.42 & anything numerically higher, 31 spline
***The 31 spline axles were offered in late 67-68 cars only.

Original GM gears were 2.56, 2.78, 3.08, 3.23, 3.42, 3.91, 4.33, 4.66, and 5.00.

There are aftermarket gears that are made to fit the 671 carrier in 3.42, 3.90, and 4.10 ratios.

You can find aftermarket carriers that are Ford 8.8 carriers that are machined to fit the 3.08-3.23 series gears.

And that is about it.

So if you want to swap to more highway friendly gears, you will need gears and the appropriate carrier, or as said above, a complete rearend from another 68-72 A-body car.

 

Where are you located ?

 

Provided you have good oil, a working oiling system, and a working cooling system, 60 mph at 3000 rpm will not hurt the engine. You'll be 3500 at 70. Unless you are in the Great State of Texas, you should be able to drive indefinitely at the speed limit on the interstate in 95% of the country. I'd keep the car in good health, cruise at 60 unless people are up your butt, then do 70 till they leave you alone and count it good.

I'd make sure the radiator is ungunked and go for it.

 

Where are you located? I have an open 3.08 rear end out of a 69 Cutlass available.

 

Thanks Retro Ranger for your suggestion. I’ll reach out to monzaz to see what would works. Ideally I would like to use the original rear end & just change out the carrier & gears. The least amount of work as possible.

 

Hello Kenneth,
Thanks for the great information. Based on this info. I assume my car should have the #588 carrier, 28 spline, with 3.91 gears since it’s a 70 car?
I really don’t want to swap out the whole rear end, I would like to find carrier/gears that could change out using the original rear end already in place.
If I understand this correctly. If I had a 671 carrier it could use 3.08 gears? & 671 carrier will fit in this rear end?

 

Hi Bob, I’m located close to Seattle WA

 

Hi Danktx, I’m located close to Seattle WA. Thanks for the offer, but shipping a whole rear end across the country? Unless you know someone over here. Or coming out this way they can deliver? I’m open to suggestions.

 

Your right. Check with lemoldsnut he's in OR. and always parting cars

 

Any of the 28 spline O-Type carriers will fit in your current rearend - the only ones that are different in that respect are the 31 spline units as the carrier bearings are larger diameter.

The least amount of work is to swap a complete rearend into the vehicle. I have done both many times over the last 40 years and it is a LOT easier to swap the entire rear than to change gears in the existing rearend.

 

If it were me I would opt for 342 gears, so as not to take a lot of the off the line performance out of the W30. I would try and leave the rear end assembly alone and swap in one with the gears I wanted.

 

That would be my suggestion as well. Does the OP really want to put USED 3.08 gears in his Type-O? Get new Richmond 3.42 gears and a new posi from Supercars (it will be a new Ford 8.8" machined to fit the Type-O, be sure to get the one that fits the AFTERMARKET gears), freshen up w/ new bearings and seals all around and he'd be good for 150K miles w/ regular lube changes. You don't want to put an OPEN rear in a W30 either, that'd just be shameful.

 

Unless I did my math wrong, or OP wants to cruise at 80, not 60, the engine speed is not that high.

 

Good Morning Koda,
Correct I would like to cruise @ 70-80+, knowing the big block has room to breath & @ anytime punching it to pass over into the left lane. Going @ 55 in the right lane with semi’s passing you is frustrating. The consensus from the group, seems to be swap out the rear end with a rebuilt rear end with correct gears, bearings, etc.
Thanks for your input.
ABF

 

Your car is a valuable piece of history.

Having said that, I would go with the plan suggested above -- go ahead and swap out the entire rear for one that is tailored to your needs. A 3.23 would probably be about right.

However, you need to make sure you keep and preserve all the parts you're replacing so as to retain the car's value. This should go for any modifications you may make to the vehicle.

 

Yes, if you swap whole rears, save the original rear, So when it is time to sell, you have the numbers matching, original rear end.

I know it is pricey, but a gearvendors o/d gets you 6 forward gears.

 

Thanks for the feed back. Gearvendor o/d is the name of the product & it would be installed into the original automatic tranny? Leaving the rear end gear ratio as is?

 

It replaces the tail shaft housing and hangs off the end of your transmission.
So you'll have
1st gear,
1st o/d
2nd gear
2nd o/d
3rd gear
and then o/d
gear splitting and o/d.
6 gears

 

I have heard that floor board mods are necessary to run a gear vendors in an A body. Is this true?

 

There doesn't seem to be a consensus, but, now that you say you want to cruise 70-80+, which is not 60 mph mentioned in the original post, perhaps you need to make a change. No one has spoken about the transmission, I am going to assume it's an automatic, and being stock, should be a th400. What I would consider to be the best solution would not be change the rear end, but to change the transmission. If you gear down the rear end, you will miss the performance that your car had at low speeds. It will feel lesser to you. I don't know if that matters, and it's only something you can decide.

Whereas, if you had an overdrive transmission, you would keep all the low end performance, and simply have an overdrive, that you could lock out with the shifter any time you felt necessary. The gear vendors unit will certainly do that, and live, but requires floorpan mods and a lot of work. If you took a performance built 200-4R, you wouldn't have to hack the floor and the car would be lighter. A normal 200-4R will die real fast behind a 70 W-30 stock engine, so it would need to be built strong, but I think it would be your best performing, least molesting, choice that would allow you to put the original transmission on a stand, preserve value, get you the cruise you want, and I THINK it will be a little faster off the line due to the different first gear gearing, but someone will have to correct me on that.

https://classicoldsmobile.com/forums...5-rear-116063/

 

Hey Koda, thanks for the reply & the link with some good reads. Lots to decide now. Everything from swapping out the rear ends to a new transmission with O/D.
Maybe I’ll leave it alone & trailer it to shows? But that just doesn’t feel right.