Anyone out there know if there is anyone offering a gear set any higher than 3.08 for the GM 12 bolt I.E. 3.00 or 2.78, something good for highway cruising to drop the RPM,s.

 

I know the factory had 2.56's as I had that in the '69 12-bolt that I narrowed and built for my '65. If I hadn't already scrapped them I would give them to you. I have 3.08's in my '68 and drove it from Lake Erie to the Smoky Mtns. and back rolling 75mph and didn't think it was bad at all FWIW.

 

You got to know if its a Chevy or Oldsmobile rear end.

 

Yes, BTW that was a Chevy 12 bolt.

 

So help me out here, new to the Olds business, mine is a 1970 442, 4 speed, original running gear, 3.23 rear gears, is the Olds 12 bolt different from a Chevy? Have found 3.08 gear sets for GM 12 bolt available on-line for sale.

 

Not only is the Olds axle completely different from a Chevy axle, it isn't even a 12 bolt axle. The Chevy 12 bolt uses 12 bolts to hold the 8.875" ring gear to the carrier. The number of bolts on the cover is purely coincidental. The Type O axle used by Oldsmobile uses TEN bolts to hold the 8.5" ring gear to the carrier. NOTHING interchanges between the two axles except the brake backing plates and drums.

 

Changing gears from 3.23 to 3.08 is a waste of time and money. Way easier to swap to a taller rear tire to lower rpms while cruising.

 

That was my exact next answer!

 

Specifically, that's a whopping 4.6% reduction in engine RPM at a given speed. As noted, changing from a 26" tall tire to a 27" tall tire gives you a 3.9% reduction in engine RPM.

 

Yeah I wasn't expecting a great deal of difference with the 3.08, was hoping to find something higher, but does not appear to be anything out there, anyway I just bought new tires, sooo guess I'll live with it, as is, awhile, anyway thanks for the advice and the learnins.

 

You might want to consider a 200-4R trans. The 0.67 OD turns that 3.23 into a 2.19:1 axle, while the 2.74 first provides more off the line than the 2.48 or 2.52 first found in the three speed automatics.

 

Yea if your going to change some stuff might as well make it count in a big way.
Would buying a new set of tires again still be cheaper than gears in the rear end?
But if your already using a 225/70/15, thats pretty tall already.

 

I did consider the auto overdrive option, but kinda hate to change out the 4 speed and considering the amount of money I already sunk into this car, anyway I go, it is gonna be a good bit of change, as for the tires am running 225/70/14 all around, so I would have to buy two 15" wheels and new rear tires. Am about ready to say calf rope and just put some miles on it. Just spent a bunch on front end and new shocks, and exhaust.