[GeeTeeOhh]

I'm thinking about changing out my '66 GTO open dif rearend for one with a 3.23 gear ratio rearend out of a '65 GTO.

Question: I don't know much about the gear ratio options. My car has a fairly stock engine with Edelbrock RPM intake and carb. I'm basically restoring this to be a fun street car. Would this ratio be a good option?

How can I verify the gear ratio without pulling the cover? I suppose I would be stupid not to take a look at the gears.

Also, is $750 a fair price for a good used rearend?

[Navy Horn 16]

When you say "rear end" are you just talking about carrier or the entire rear axle assembly?

If you are just swapping carriers, you need to make sure they are the same size (likely both 8.2 inch 10 bolts). If I were looking to upgrade rear ends from your open differential, I'd get a new posi unit and gears between that 3.23 and 3.42.

I wouldn't pay $750 for a used carrier/gears. You can get new parts for that much.

[RocktimusPryme]

You can rough calculate by spinning the yoke and counting spins of the axle/wheel. a 3.23 should spin the tire around twice in roughly 3 and a quarter turns of the driveshaft.

[ZeGermanHam]

If I were purchasing a used axle housing & differential, I'd prefer to remove the cover and have a look at the gears and count the teeth to verify the ratio. That is the most surefire way to confirm you are buying what you think you are.

Otherwise, you didn't mention which transmission you have installed, but a 3.23 ratio should be pretty livable with pretty much any transmission for a street cruiser. If you want to get a sense of how it will affect your RPMs, do a Google search for engine RPM calculators. There are a bunch available, and after entering your diff ratio, tire size, trans gear ratio, and vehicle speed, they will tell you what your RPMs will be.

[RocktimusPryme]

Honestly the more I think about the financial aspect of it, unless this new rear end is an upgrade to yours beyond ratio. A 9 inch, for example, or maybe it has new disc brakes on it.

For less than 750 I feel you could have a new limited slip installed with your ratio of choice. Not be using used parts.

[GeeTeeOhh]

good points all. This is for an entire rear axle assembly. One plus that I can think of is that it is supposedly 1" narrower than the '66 rear, which would give me some more clearance for the rear tires I plan on installing. It is a BOP 8.2 10-bolt cover, casting number 9779822 matches up for a 65, axle code YF matches up with 3.23. I would definitely want to pull the cover. It's got a M-20 4-speed.

[JLMounce]

Originally Posted by RocktimusPryme

Honestly the more I think about the financial aspect of it, unless this new rear end is an upgrade to yours beyond ratio. A 9 inch, for example, or maybe it has new disc brakes on it.

For less than 750 I feel you could have a new limited slip installed with your ratio of choice. Not be using used parts.

I'm inclined to agree with this. I had the 10 bolt in my wife's chevelle redone with a posi, new axles, ring and pinion etc. and it wasn't much more (including labor) than that $750.00 you're looking at for a used axle of unknown condition.

If that axle had recently been serviced, you know exactly what ring/pinion is in it, you know that the posi unit is new or newer with good clutch packs etc. It's probably worth that $750.00

If it's old, there's no info on it, ratios are unkown, last service on the posi unit is unknown, you should expect to have to replace some or all of that, making it's value much less.

[geeteeohguy]

Originally Posted by GeeTeeOhh

good points all. This is for an entire rear axle assembly. One plus that I can think of is that it is supposedly 1" narrower than the '66 rear, which would give me some more clearance for the rear tires I plan on installing. It is a BOP 8.2 10-bolt cover, casting number 9779822 matches up for a 65, axle code YF matches up with 3.23. I would definitely want to pull the cover. It's got a M-20 4-speed.

Been driving GTO's since the late '70's, all gear ratios. IMO, the 3.23 is the best all around rear gear for these cars with a stick. It is fine off the line, and good on the highway at high speed. $750 for an open rear end is pricey, if it is an LSD it is right on the money. These are not strong rear ends, but driven within design limits, work just fine. Both my GTO's have run hundreds of thousands of miles on them over the past 40 years. Don't drink the Kool-Aid that you need 500 HP and a Ford 9" for a cruiser. You don't.

[GeeTeeOhh]

Originally Posted by geeteeohguy

Been driving GTO's since the late '70's, all gear ratios. IMO, the 3.23 is the best all around rear gear for these cars with a stick. It is fine off the line, and good on the highway at high speed. $750 for an open rear end is pricey, if it is an LSD it is right on the money. These are not strong rear ends, but driven within design limits, work just fine. Both my GTO's have run hundreds of thousands of miles on them over the past 40 years. Don't drink the Kool-Aid that you need 500 HP and a Ford 9" for a cruiser. You don't.

Thanks for the insight. The assembly I am looking at has a LSD. and if it checks out may think hard about buying it. Any recommendation on what else to look for, or what can be checked out beyond taking off the diff cover?

[Navy Horn 16]

Originally Posted by GeeTeeOhh

Thanks for the insight. The assembly I am looking at has a LSD. and if it checks out may think hard about buying it. Any recommendation on what else to look for, or what can be checked out beyond taking off the diff cover?

Just because it has one in there doesn't mean it is any good.

Look and see what what just clutch packs for old school posi units cost (about $160), plus the cost of having it rebuilt, plus the cost of having the whole thing gone through...It's cheaper to get a new LSD carrier and whatever gears you want. Probably by half.

[bdk1976]

What ratio are you running now?!

Also, $750 is a decent price for a complete '65 posi 3.23 axle around here. I'd pay that if it was in good shape and what I was looking for.

[TROYBOYONE]

3 years ago I bought a 3:08 posi complete used rear end including the housing/axels, new brake shoes ,wheel cylinders, and brake hardware for $500.

[lust4speed]

Pontiac made the change to the slightly wider rear in the middle of the '66 model run -- so the width might already be the same as the mid-66 and earlier. Late axle flange to flange is 60-15/16" or close enough to call it 61", and early will be 60" in round numbers.

Early cone type limited slip units work great - right up until they don't. Check by leaving one wheel on the ground, trans in neutral, and jack up other wheel and see whether you can turn it by hand. Goes without saying to block the front wheels so you don't have to chase the car down the block. A good posi unit will have about 250 foot pounds or more of resistance and it should be very difficult to turn the raised wheel with hands across from each other trying to spin it. If it turns you would be much better off putting the money for new parts in your current unit.

Around here $750 would be a normal price for an old LSD equipped rear with no slippage and no signs of leakage, and probably still needing new brake pads and turned drums. A new limited slip in a cleaned up and painted rear set up properly would be at least $1,000.

[geeteeohguy]

Originally Posted by GeeTeeOhh

Thanks for the insight. The assembly I am looking at has a LSD. and if it checks out may think hard about buying it. Any recommendation on what else to look for, or what can be checked out beyond taking off the diff cover?

Yes. Sounds like a good deal. Try to wiggle the axles up and down at the flange and turn the axles...any up and down movement or roughness means it needs new outer bearings. Common. Have a buddy hold one axle while you try to turn the other. If you can't turn it, the posi is still good. If you can turn it, it is getting worn. If the axles rotate opposite directions when the pinion is turned, or only one axle turns, it is an open rear end, either by design or because the posi unit is worn out. These cone type units can be rebuilt.....Larry Woltzen of CA differential (Now in Illinois) did one for me 30 years ago that's still going strong.