[leeklm]

As some have seen in other posts, helping a friend with his 66 GTO, which has a fresh 389 tripower build using original non ported heads, 9.3 CR, and the 218/224 cam shown in the attached pic. This build had 340hp on the engine dyno, but that was in cold air with full length headers- so of course lower numbers in the real world with soon to be installed RA manifolds.

The car has a BW T10 with the 2.64 1st gear and 3.55 rear.

Based on his non-racing driving style and a desire to cruise the highway, i was thinking of a swap to 3.08 gears. We would have a 1st gear final drive ratio of 8.13. I was happy with an 8.3 1st gear final drive in my 66, but that was a considerably higher output motor.

A 3.23 gear would be ideal, but this is the original 8.2 rear, so assuming 3.23 is not available?

Since the car can currently start reasonably well in 2nd gear on level ground, I am thinking the 1st gear ratio will be ok with the 3.08. My concern is overall driveability - will this engine/cam combo feel sluggish at the lower cruising rpms, or since it is a manual trans, maybe that can be easily countered by simply shifting to a lower gear as needed.

Thoughts?

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[track73]

In my opinion no 3,08. I had installed a T-10 with the close ratio 2,20 first gear in my Transam and first gear starts were not easy. with the 3.23 rear gear. I rebuilt the original 2,64 trans and reinstalled it. My 400 engine was similar in power to the 389.

[leeklm]

Originally Posted by track73

In my opinion no 3,08. I had installed a T-10 with the close ratio 2,20 first gear in my Transam and first gear starts were not easy. with the 3.23 rear gear. I rebuilt the original 2,64 trans and reinstalled it. My 400 engine was similar in power to the 389.

Keep in mind the 1st gear final drive ratio you mentioned with 2.20 & 3.23 gears is only 7.1. The 8.1 final ratio in our example falls somewhat in the middle of your 7.1 and where we are now at 9.3.



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[JSchmitz]

I would stay with 3.55 personally. What size rear tire? Could he go up a little in height? That can make a big difference.

[b-man]

Find a good used 3.23 gear.

He will quickly grow to dislike the 3.08 gears in city driving.

[Tom Vaught]

The 2.64 Trans first gear and the 3.08 axle with the right tire diameter will be fine and live a long lift as far as clutch wear BUT You did not say in Minnesota where you actually lived, (in the country or in the city) and the 3.23 has benefits in city driving. Just like that 4.33 gear had with the tires I ran.

JMO

Tom V.

[leeklm]

27" tires. I did not realize used 3.23 gears were available for the 8.2 BOP. I will dig into that...

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[Tom Vaught]

He already has a 3.55 gear set rear axle in the car. And a different Posi unit vs a 3.08 gear has. You can use the 3.23 posi unit on 3.23, 3.55 and 3.90 gears. Even 4.33 gears.

Put the 3.23 gears in the car based on this new info.

Tom V.

[b-man]

3.36 is the tallest gear that will fit the 3.55 posi.

He needs the next one that fits 2.93, 3.08 and 3.23 gears.

[JSchmitz]

I have 3.23's with 26" tall tires. I liked 2.5 first better than 2.2. That's with a 455 in a light car though (3,600#). With lesser cubes and heavier car idk. I think I'd want those 3.55's. I had a 389 in a '64 with 2.72's when I was a kid. It did fine even with a 400 turbo. Factory cam though.

[padgett]

Depends on what you want to do, My 72 wagon had a 3.08 posi and 28" (H78-15) rear tires behind a smogged 400 4bbl and THM400. Was a great trip/tow car but not very fast off the line. The Goat has a warmed RA 400/4-speed and a 3.55 4 pinion. Would like an OD for anything over 60.

So first decide what it is going to be for, then pick a ratio.

Am curious: what ratios were available for a four pinion ? Thought it was stronger than a standard axle.

[AIR RAM]

In my personal opinion for street driving/performance… (best of both worlds) You should aim to be at or above 2000rpms @60mph… I have found that to be a sweet spot for all round driving to include optimal MPG. To low of a gear tends to bog the engine down creating the need for more fuel to do the work. And going to tall creates higher RPMs which can consume more fuel.

Aiming for 2000RPM in top gear will get you the best of both worlds in most cases.

Apply the same principle to overdrive transmissions as well.

[padgett]

Just a little quibble but for best MPG with a gas engine you want to run about 2,000 rpm at 70 mph (most interstates and toll roads around here).

Why I kinda sorta want a OD trans for my Judge with 3.55s. Problem is twofold:
- 450+ lb-ft of torque
- am Scots

[RocktimusPryme]

More extreme than what you are talking about, but I recently went from a 3.50 gear in my 4 speed Galaxie to a 2.75. All I had in my mind was cruising in 4th gear. In truth, it made 1st gear starts from a stop much harder. Im afraid Im going to eat the clutch. It also started pinging in places where it didnt before. Probably a factor of a heavy car, a touch too much compression, and too little gear. I had to take timing out of it when I swapped in that highway gear.

I also found that Im rarely on the highway in that car, so most of the driving I do is now done in third gear. So I didnt even really gain any economy out of it.

A 3.25 would probably have been the correct compromise.

I think with an auto car I would have gotten away with it, but the manual not so much. 4th gear is nice though with that 2.75 on open highway.

[i82much]

I thought most of the M20 cars came with a 3.23 rear originally?

[steve25]

Motors like the 389 and 400s with there 3.750” stroke in cars of these weights love 3.55 gears, so I would not change a thing!!

[Tom Vaught]

Originally Posted by b-man

3.36 is the tallest gear that will fit the 3.55 posi.

He needs the next one that fits 2.93, 3.08 and 3.23 gears.

COMPLETELY FORGOT about the 3.36 gear being at the bottom of the offerings for the 3.55, 3.90, and 4.33 rear center section. I knew there were 4 gear offerings and assumed the 3.23 gear as the 4th one. Sorry. Thanks Bart.

Odds of finding a 8.2" rear axle 3.36 set of gears will be a challenge and not worth the bang for the buck to swap out the whole rear axle for the 3.23 and lower number axle ratios.

Better money spent installing taller tires to affect the engine rpm as speed.

Tom V.

[b-man]

Originally Posted by i82much

I thought most of the M20 cars came with a 3.23 rear originally?

Had a ‘69 400 Firebird factory equipped that way.

[i82much]

Originally Posted by b-man

Had a ‘69 400 Firebird factory equipped that way.

that's how my 65 GTO four barrel was originally equipped. i broke the 3.23's, went to 3.55's, broke the 3.55's, went to 3.90's. at that point my bank account was broke too so i started driving like i had some sense and stopped breaking rear ends, lol.

[padgett]

Thought 3.23 came with AC cars mostly. My Judge did originally have an open 3.23 but thought that was for AC but might have been for M20. A really nice 3.55 4 pinion came along and I jumped on it.