I have a open 12 bolt o rear end . The motor is new rebuilt stock tranny auto. Put 295 60 15 tires on it. There is a guy that is selling a 12 bolt posi with 391 out of a 69 olds hurst car.is 391 to low for the freeways? will this bring up my rpm to high? He wants 1000$ for the rear end. So what gears should i go with ?

 

There aren't that many original 3.91s around so they do command top dollar, in my opinion. A 3.91 with your THM is going to run up your RPMs and drop your gas mileage on the freeway. When you get on the freeway you'll be able to come up to speed pretty quick, though.

I've always liked the 3.23 or the 3.42. I've always thought of the 3.91 as a racing differential and not real conducive for freeway speed type driving.

Just my personal opinion.

Randy C.

 

X2 what Randy said. Its a neat rear end but on a long trip I think the rpm's will be uncomfortable.

If you can afford that price range I'd encourage you to talk with Brian, 507OLDS. He's got a couple rebuilt rear ends for sale right now. John


https://classicoldsmobile.com/forums...23-3-42-a.html

 

That rear end is a valuable piece for someone with a car missing the correct coded differential. I would not pass it up or if you do help one of our members get it.

Lee

 

Don't think you'll be much liking the 3.91 on the freeway. My car has 3.73's with slightly shorter 255/60/15 tires, and about 3500 rpm at 70 mph with converter slippage. Changed over to a 700R4 0.70 od lock up tranny and about 2400 at 70 now. Really the way to go for highway cruising, as like running a 2.61 axle in top gear.

 

My quick calculation shows 2,725 RPM at 60 MPH, which is actually less than I expected (and so might be wrong ).

I'd try driving down the highway at that speed to get a feel for what it's like before making any decisions (2nd gear ought to do it).

- Eric

 

Your calc's look correct...

If the OP has a 2.56 now, the RPMs in 2nd gear with a TH350 (1.52) would be nearly identical to 3rd gear (1:1) & a 3.91.....

 

A set of 3:42's will slip right into that carrier.

 

So dose eanybody have a posi rear end or carrier with gears for a 1969 12 bolt for sale that will not cost 1000$ i want to stay with the 12 bolt if i can?

 

Be aware that your Type O axle isn't a "12-bolt". There are only ten bolts holding the 8.5" ring gear. A true Chevy 12-bolt has 12 bolts holding an 8.875" ring gear. The Type O is factory correct, but also rare and difficult to get parts for. Chevy 12-bolts were factory installed in Canadian-built Cutlii in the 1968-70 timeframe.

 

Yes, it's an o axle 12 bolt 256 gears 1969 cutlass and yes it is hard to find parts. I don't know much about rear ends but last week went to the junk yard and got a 8.5 out of a 1976 442 got it open and it was a one wheel wonder 256 gears just my luck and now i am weighing my options as there are not many oldsmobles around here.

 

Try Supercars Unlimited (www.supercarsunlimited.com) near Portland, OR, and look at their on-line catalog. They have replacement differentials (3.42 posi or 3.91 posi) for 28-spline axles that will fit into your existing differential housing. But, again, we are talking cost. Either set-up will cost about $1000.

Randy C.
Olympia WA

 

Like was said before. Get a hold of Brian Trick, for he has all the parts you need or can sell you a complete. Maybe even make a deal for your rear in trade and some money for a complete. He is here sometimes and on "Real Olds" power.
Do a search.
Gene

 

I did talk to him and to ship me a rear end will be 1800$ that a little much for me. That why i need to find someone in Washington that has one .called all junk yards no one has a 69 found one in yakima 100 miles away and now i have it a 8.5 10 bolt rear end 256 one wheel payed 200$ for it . hope i can find another used rear for maybe 500$

 

I too would like to say the 3:91 gear will probably be too steep for a lot of highway use. I have a 69 442 with the 3:91 gears I use it when I a go drag racing. I also have a rear with 3:23 gears that i put in for cruising It take me about 1 and a half to 2 hrs. to change it out. I really like the 3:23 for the road. Keep checking on E bay . I see them on there every now and then.

 

Keep an eye out at 442.com as they pop up there also.