Rear gear and transmission choice
 

The 4 speed in my '69 GTO bit the dust, which is not surprising since it was a 3.44-first-gear ST10. The existing rear end is a standard (non-posi) 12-bolt 2.73, which worked decently well with that oddball 4-speed.

I have an 8.2" 10-bolt with 3.36 posi I pulled out of a 69 firebird I wrecked (sadly, got rear ended very hard), or I could build the 12 bolt. Seems like I'd rather throw that 10-bolt in there and see how long it lives. It's rather whackadoodle, with a weird repair on the carrier - I'll try and attach a picture - but it lived just fine in that firebird, behind this same 400.

Not sure how much power I'm putting into it - this is a plain jane 400 with that stupid down-in-the-hole piston kit, #16 heads off a 68 gto I think (only lightly modified, unported. Big valve D-ports.) an ultradyne 223/231 cam if I remember from many years ago, and a performer RPM intake. Currently no headers, maybe later. Runs nice, should be good with a wide ratio transmission.

Future plans are a nice 455 with either ported 6x heads or edelbrock heads or something. Hopefully way in the future when this 400 is broken.

No plans to ever throw slicks on it or anything. It may get a few runs at test and tune just to see, but this will mostly be driven half the year daily.

I think I've settled on a TKX transmission. The 3.27 first gear and .72 overdrive sound nice, but the close ratio should also work well enough with 3.36 gears. So, my questions are, is this a good plan, or would you guys like to talk me out of anything here? 3.36 8.2" 10-bolt from the firebird into the rear (yeah, some modifications needed) and see how long it lives, and the wide ratio TKX. Is that first gear gonna be a granny gear with a 3.36 and stock-ish tires? I run 255/60r15 currently.

If that's a bad plan, plan B is to build that 12 bolt with 3.73 posi and get the close ratio TKX (2.87 first and .68 overdrive), but it'd sure be a lot cooler and cheaper to use up that old rearend and just keep the 12 bolt to build if I ever break it.

Might like a little taller first gear.I ran a 3.27 with a 3.31 in the back of my 69 455 bird.First gear was REAL quick and shift.Tom

Here's a great gear ratio calculator/comparison tool.

What size rear tires?

https://www.blocklayer.com/rpm-gear

I played around with that a bit, not sure what exactly I'm going for though. 26"-ish on a 255/60 I believe.

Thanks Tom, so if you did it over again you'd go with the taller first gear... Too bad I can't order both and play around with it!

Or a taller rear gear?Tom

I've got that 2.73 peg leg already in the car, but, uh, I doubt you mean to try *that* tall :p

I run a 2.68 with a 3.42 ST-10 in a 62 421 Grand Prix.Tunnel Ram with a pair of 750 AFBs.No slipping the clutch to get the car moving and great on the highway.FWIW,Tom

I'm running a moser 12 bolt with 3.73s and a TKO600 with .64 final drive in my 67. 1st gear is pretty short before you hit redline so I have to keep an eye on that and cruising at 70 is around 2500rpm. My rear tires are 255/45ZR17s.

That F body rear does not have the needed control arm brackets to bolt into the A body you know!

Youve got other hurdles first

How are you going to fit a 1st gen leaf spring rear in a 69 4 link a-body? Not only that but the 1st gen f-body rear is narrower than what your GTO originally takes. By about an inch or so.

You're better off just getting a carrier and gear set for the Chevy 12 bolt and keeping that in the car.

Indeed. I've already tapped out on doing this project myself - I am getting a bit lazy lately, but probably would have attempted the transmission conversion if not for this rear end issue as well - and if we go that route the mechanic's plan is to measure and weld. I'll bet it takes him a few tries.

I figure if one inch kills the build, I was already in trouble...

I was thinking if I go with the close ratio transmission and the f-body rear, I can save that 12 bolt and build it with the correct gearing for whatever the next engine is. That 400 is a 20+ year old build and has to be around 100k miles by now. Maybe it'll outlast that rear, maybe it won't.

I am open to being convinced to just do the 12 bolt now and get it over with, though.

Me personally I wouldn't want to muck up a 69 GTO with a cobbled f- bod rear, especially when it already has a perfectly capable GM 12 bolt already under it. It's way stronger, do it once and be done.

Just go with a 3.55 gear, it'll work with either tkx you choose and works well with any larger engine you might build in the future.

Me personally I wouldn't want to muck up a 69 GTO with a cobbled f- bod rear, especially when it already has a perfectly capable GM 12 bolt under it. It's way stronger, do it once and be done.

Just go with a 3.55 gear, it'll work with either tkx you choose and works well with any larger engine you might build in the future.

I have a stock HD 10-bolt with factory 3.90 gears. I run a TKX close ratio (OD is .68) and a GR70-14 tire. Yes, first gear is done before I clear an intersection, but even with small tires/short rear gears, I can cruise 65-70 all day long and never see much more than 2200rpm. The TKX is an expensive swap, but no other single modification can match it for making an old car more enjoyable!

Plain and simple, the Firebird rear end isn’t economically feasible to convert into something that fits into a GTO. The cast iron center section doesn’t have the mounting ears for the upper control arms, why anyone would think it’s a good idea to try to weld something that’s highly stressed to that is beyond me.

You already have a far superior 12-bolt in your car, just change the gears to something that best matches up with the overdrive transmission you’re planning to use and add a posi unit.

The 8.2 rear end is inherently weak, sell it to someone who needs it for their early Firebird.

If you stay with your 12 bolt with the 2.73's and go with the TKX with the 3.27 first gear - you end up with an effective 1st gear of 8.9271. That will work fine from the stop light.

Your 255/60's (assuming they are 15's) are just over 27" tall.

With your .70 overdrive - that puts you at 63 MPH at 1500 or 84 MPH at 2000 RPM.

Nice cruising RPM.

Or, if you want, I have a set of slightly used (less than 10,000 miles on them) 3.42 rear gears for a 12 bolt available is you want. Then you could go with one of the other TKX's with a more conservative 1st gear and a taller OD.

Is it a Chevy 12 bolt or an Olds 12 bolt. They’re different. The F body rear won’t work in an A body. No top trailing arm ears that are cast into the housing like an A body. If it’s a Chevy 12 bolt you’ve got a strong rear axle with endless parts available.

Update for when somebody finds this via search - after the last few responses, it was clear they were correct and the f body rear is not something I want to try and transplant. Going to build the 12 bolt.

12 bolt with 3.73