3.55 or 3.73
 

It's been awhile since I have been on PT and longer since I have posted. I am not the builder of my car and the car has been pushed aside for awhile now. It's time for some forward progress. I know what my builder wants but would like to get everyone's opinion about a rear gear. Have a 1969 Firebird. Will be a street car and would be fun to take it to the nearest road track a time or two (Bowling Green). Will have a Ford 9 inch (DSE), 31 spline, 335/30/18, T56 Magnum and probably 600+hp. I know there are several calculation sights out there to help me with a set up but wanted to hear from all of you. 3.55 or 3.73? Please educate me.

Thank you for your time.

Ron

3.70 is perfect with a T56. Make sure you get the 2.66 1st gear in the Magnum.

For sure the 3.70. I had 3.50 gears in my '67 with an LS/T56 and 6th gear at 70 mph lugged around 1500 rpm. You'll been much happier with 3.7x gears.

370 or even 390 with a 6 speed.

Agreed, I had 3.73 with my t56 and rarely got it into 6th. Ended up going with 4.10, partly b/c going 25 mph in my hilly neighborhood left me right at a point where I was constantly shifting.

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4.11 on my T-56.. power band is perfect.

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Should I go with a 5 speed instead?

The t56 gives you bragging rights, but unless you're putting down tons of power a 5 speed is perfectly fine for street driving and will save you from having to cut the trans tunnel in most cases.

I have a TKO 600. IMHO I would of never used the 6th gear. 5 speed and 373 gears with the .064 overdrive and your good to go.

Any low lugging rpm's with the 4.11?

What do you think about aT56 with a 4.56 rear gear and a 2.96 tranny gear?

Gears
 

I have a tko 600 5 speed with 411's

gear ratios are 2.87 too low for me... 1.89 1.28 1.1 and .64
the .64 is too far away from the 1.1 4th gear to me
either 3 or 4th can't remember howl like crazy

I am going to change the rear gear to 3.73 but I think first
will still be useless and the gap from 4th to 5 still too much.. maybe the .83
would have been better but they said go .64

Wish I would have keep my old 4 speed as the ratios were perfect
except 1.1 was too low going down the freeway but everything
else was great for crossing, street driving, etc

Just my 2 cents

Bob Benson- Dreamin Ta's 70 firebird

Here is why you want the T56 Magnum.....

The 2.66 version is basically a close ratio 5th speed (.80 5th) with .63 6th gear. Truly the gearing is primo. I would like a little more overdrive... but oh well.
They hold 1000rwhp in stock from. I'm pushing over 1400hp with my 100% stock unit shifting at 7800rpm in 1-5th gears without ANY issues running a stock slave cylinder and RXT twin disc. 5th gear is holding about 1400hp without issue. This is in a lightweight 2900lbs car though (LS9 turbo RX-7).
It modernizes the car with how smooth they shift once broken in. That alone is worth the cost.
You can change out 5th/6th gear if you want ZR1 gearing or other ratios....
Abundance of replacement parts in stock (older T56's have parts backorder issues)

The TKO600 was designed many moons ago and doesn't shift well at high RPMs.
4th is 1:1 and opting for the .64 5th gear the RPMs fall off a cliff
Shifting smoothness is not even in the same ball park as a T56 Magnum

It is much smaller though and about 50lbs lighter as an attribute.

Gearing will depend upon how much power you plan to make... and redline....
here is a good calculator to play with to figure it all out ;)
http://fitbodyshop.net/gear/index.php

I have a 4th gen LS T56. I used to get a little bit of lugging woth my 3.73s, but now i have 3.90s and its right where it needs to be. Very happy.

4.56x2.96=13.5, you wont get much use out of first gear.

Add to this that with that type a gear at high speeds you start getting into drive shaft critical speed and risk it coming apart in an impressive manner.