[markpj23]

Getting ready to build that 428 to drop in my soon to be 2+2 clone. Building the right engine also means building it to 'fit' the rest of the drive train and that brings me to the rear end.

Axle tube now says UC code which equals 2.56 open if I've read the charts correctly. I also have very little history on this car, and the seller indicated the rear end was a posi unit. So pulling the cover to inspect is on the agenda.

Driving the car now with no tachometer, highway speeds are 'comfortable' from an engine sound perspective and performance from a standing stop is not real impressive. That leads me to believe that the original highway gears are still there.

I know that choosing a gear ratio is kinda dependent upon how the car will be used, and this will be a local cruiser not destined for lots of interstate driving. I hope to make a few out of state rally events, but for the most part the car will be an around-town daily driver.

My hope is to get decent performance off the line and not feel like the engine is working too hard at highway speeds. Right now I'm not sure if I could light the tires if I wanted to.... which I kinda do

So with the 2.56 gears I believe I can go to 2.93 with just a gear change. Wondering if that will be enough of a performance ratio, or do I need to bite the bullet and get the new carrier and 3.23 or 3.42 ratio?

I believe it's important to get this resolved before the engine build so that I can make sure the power & torque curves are right for the car. Hope I'm not overthinking this one... so let me know.

[67Fbird]

I believe that 2 carrier goes up to 3.08?? but even that is a bit TALL. Used to be able to get a "thick" 3.23" gear...for just this issue. Some will not like this remark...but you can go 3.23-3.42 with a spacer. If it is a dog leg....go ahead with a carrier (just not Auburn).

[tom s]

Is a OD trans in your future?Make that decision first IMO.Tom

[Stuart]

What transmission, automatic or manual? You can look at a 1967 Performance brochure here, it lists the various rear end ratios recommended by the factory for a 2+2: http://oldcarbrochures.org/United%20...ure/index.html

[ANDYA]

If your looking to keep the long tail muncei , Autogear makes an M22Z gear set. First gear is 2.98 and fourth is 1:1 so you can run 3.08 or 3.23 gears. You get good performance off the line and also good highway cruising rpm.

[markpj23]

Originally Posted by tom s

Is a OD trans in your future?Make that decision first IMO.Tom

Hoping to avoid the expense / hassle of a transmission swap or gear vendors setup.....

[steve25]

Note that the factory also had a reason for gear ratios below 3.55 and that was if the car had A/C so the average rev’s would not harm the compressor, no less peak rev’s!
Car’s with optional ram air engine packages which came mandatory with 3.90 gears, or cars ordered with 3.90 gears and up could not be had with A/C.
So does your car have A/C?

[markpj23]

Originally Posted by ANDYA

If your looking to keep the long tail muncei , Autogear makes an M22Z gear set. First gear is 2.98 and fourth is 1:1 so you can run 3.08 or 3.23 gears. You get good performance off the line and also good highway cruising rpm.

That's an interesting approach. Quick search looks like they're hard to find and pretty pricey too.... $2,500 for the whole transmission IF you can find one. Gear sets out of stock..

[Murf]

IDK, I realize everybody’s idea of “street cruiser” is different. My wife’s 66 GTO has a 455, 2.88 low ST 10, 27 inch tall tires & 3.08 gears. Cam is only 230/236 on 110. I would prefer more gear or a little less cam for around town driving. I think mid threes would be good but highway revs would be worse.
Good luck with your decision!
Murf

[markpj23]

Originally Posted by steve25

Note that the factory also had a reason for gear ratios below 3.55 and that was if the car had A/C so the average rev’s would not harm the compressor, no less peak rev’s!
Car’s with optional ram air engine packages which came mandatory with 3.90 gears, or cars ordered with 3.90 gears and up could not be had with A/C.
So does your car have A/C?

Yeah I picked up on that in the performance brochure. Don't have AC but am adding a Vintage / Sanden compressor unit so I don't think it's a concern.

Thanks for the heads up....

[geeteeohguy]

Not sure about the big cars, but an A body with the 8.2 rears, 2.56 and 2.78 are in a group of their own. Huge pinion gear, thin ring gear. Then the split goes 2.93 through 3.23, and 3.36 through 4.33 and beyond. I have a 2.56 posi in my '67 GTO with a de-tuned original engine and a TH400 and it fries the tires from a stop. That said, I think an ideal 'all around' gear for a big heavy car like yours would be a 2.93 or 3.08. Good around town, good on the open road. People who run 3.42 and higher gears don't really do much driving on the highways unless they have overdrive set-ups. It gets old really quickly. I have a friend who installed the stock 3.55's in his '67 GTO and he gets about 9 mpg less than I do driving 25 mph slower on the freeway. (His cruise is 55-60, mine is 75-80 and I'm using way less fuel)

[markpj23]

Originally Posted by geeteeohguy

..... That said, I think an ideal 'all around' gear for a big heavy car like yours would be a 2.93 or 3.08. Good around town, good on the open road. People who run 3.42 and higher gears don't really do much driving on the highways unless they have overdrive set-ups. It gets old really quickly. I have a friend who installed the stock 3.55's in his '67 GTO and he gets about 9 mpg less than I do driving 25 mph slower on the freeway. (His cruise is 55-60, mine is 75-80 and I'm using way less fuel)

Thanks for this. I have been thinking that a 2.93 would be the cheapest way to go since no carrier change (if I've read the B body charts right). My determining factor will be whether or not the PO added a posi unit. If not, I'll do so and get the 3.08 gears & new carrier.

[HWYSTR455]

I would say bite the bullet and go with a new 3 series carrier. That way later, you could always go to a TKO-600 upgrade and change ring & pinion.

I have seen trans-only TKOs go for $2350, and xmas is right around the corner. A manual trans car is a less-expensive OD upgrade than an auto.

If you think you may in 2-3 years be able to swing a 5-speed TKO, you may even consider going to a 3.55 or so gear, and live with it until then.


.

[steve25]

If it is 2.56 gears it’s not posi if it’s all stock.
A posi unit would have had to be put in.

[rustyrelic]

Sounds like a cool project. A four speed would be very neat. Something I've thought about many times.

I know there is much less "traffic" on the 67 and Up full size section, but you may want to move your question over there where people with direct knowledge would be more likely to respond. I assume this rearend you have located is from a full size and has the correct pick-up points, etc for locating the control arms for a 67? or maybe it is just the center?

While I haven't looked it up in a while, as others have pointed out there are different carrier series for different gear ratios and you cannot just swap gears willy-nilly. Years ago I was fortunate enough to score a 3:42 posi center section. While it works, it howls a bit. Ok off the line, with a low-po stock 400, but remember this is a heavy, heavy car ~ 4000 lbs? All out of breath at about 75 mph. Wound up pretty good on the interstate with the stock 400 hydro.

I think if it were me, I'd take a hard look at using a 4L80E auto with overdrive. Is it as cool as that big chrome shifter? Nope! More drivable with better performance, I would bet yes.

[rustyrelic]

The full size 12-bolt with 10 bolt cover for a Pontiac is unique and there is little aftermarket support. Again, years ago, I got some stuff from FabrCraft Metalworks -
https://fabcraftmetalworks.com/produ...ory/gear-sets/

No idea if they can still help you.

[markpj23]

Originally Posted by steve25

If it is 2.56 gears it’s not posi if it’s all stock.
A posi unit would have had to be put in.

Agreed. No history on the car so we're pulling the cover today to see what we've got.