michael hilsabeck

I have a 12 bolt rearend out of a 1969 cutless s I had the motor tuned up at the shop and guy said that if I put 390 gears in it that i would have more power but then i have heard other people say put 273. I have check on the 390 gears and I was told i could no longer get those gears with pose track. So i need your guys help could you give me you input.

joe_padavano

First, unless your car was built in Canada, your axle is a Type O, not a Chevy 12 bolt. If you open it up, you'll only see ten bolts holding the 8.5" ring gear. Chevy 12 bolt axles have 12 bolts holding an 8.875" ring gear.

The gear ratio has nothing to do with the amount of horsepower the engine makes, however it can help you use it better. Lower numerical gears give you top speed and fuel economy with slower acceleration. Higher numerical gears give you better acceleration but higher engine revs on the freeway and lower gas mileage. Your call as to which is more important to you. Be aware that parts and gear sets for the Type O are hard to find, so you may not have a choice.

rptw32

you also have to be mindful of what carrier is in the rear end now.
Olds made 3 different carriers each designed for specific gear sets
you can't put a 3:90 gear on a carrier that originally had a 2:56 - 2:73 series or the 3:08 - 3:23 series -

you need to find which carrier you are working with before you start to swap out gears unless you're planning on going aftermarket carrier also .

Rgds

joe_padavano

Actually, you can, so long as you use a spacer or extra thick ring gear. The spacer is not the preferred solution since it can lead to bolt bending. I believe most of the aftermarket gear sets are designed to fit the 3-series carrier, even for the 3.90 and above ratios.

Just to expand a little, as the ratio gets lower numerically, the pinion gear gets larger. This requires the ring gear to be further away from the pinion centerline, hence the different carriers. Since the 2-series carrier has the ring gear furthest away, you can space the ring gear for a 4-series gear set. The opposite is not true, since you can't have a spacer with negative thickness.

old442

If gas milage is what you are looking for go with 273 gears. If you want more get up and go 391 fits the bill . I am currently running 323 gears which is a happy medium . Good for all around in my book. The first thing I would reccomend is that you see what gear you currently have and go from there.

michael hilsabeck

on my 1969 im sure it is stock rearend the cover has 12 bolts that hold it on but i don't no what size it is. What size stock did 1969 come out with with a 350 ?

InfoJunkie

Most of the Cutlasses I've had were 2.56 with a 2bbl and 2.78 with a 4bbl with the O type axle but as Joe mentioned things are different in Canada. My 68' and 69' both had C type.

joe_padavano

Yes it does. It's still a Type O. This is a source of confusion for many people. Twelve bolts on the cover does not necessarily make it a twelve bolt axle. It's the bolts on the ring gear that matter.