Tremec TKX 5th Gear Choice

[Seven]

I'm installing a 455 H.O. into my 1972 Firebird.  The engine has stock round port heads, Pontiac aluminum intake, forged internals, mild roller cam and 2.5" Ram Air exhaust manifolds.  Dyno output was 460 h.p. at 5200 and 550 lb-ft at 3700. 

The car was born with a 350 2 barrel, 3 speed Saginaw manual transmission and 3.42 gears.

Rear tires are 27.1 inches tall, 255/50/17.

The car is a cruiser, will never see a drag strip, and is driven mostly on 2 lane highways (65 mph cruise).

I'm considering a Tremec TKX with either the .68 OD with the stock 3.42 gears, or changing to a 3.08 gear set and going with the .81 OD.  Either way the rear end is coming apart to install a TrueTrac differential.

Would the gear drop between 4th and 5th gear with the .68 OD be excessive and cause drivability issues?

[5th T/A]

I believe you can find online calculating programs that will give you engine RPM with given gear ratios and tires sizes. Your engine has a tremendous amount of torque so I wouldn't anticipate any problems. Hopefully someone here has run a similar setup with a 3.42:1 gear, 27" tall tires and will provide feedback. I imagine your car with the current engine and an open rear end, just wants to spin one tire and has terrible traction issues.

Just for reference my 1980 TA came with a 150 HP 301, a 2.41:1 rear gear and 225 70 15 tires (27.4" diameter) and had no problem pulling the car at highway speeds. Engine turning roughly 2000 RPM at 60MPH.

Currently I have a Pontiac 461, Horsepower was 489 at 5000, torque 581 at 3900. I am still running the 2.41:1 and turning about 2000 RPM at 60 MPH. Also running original turbo 350 transmission with a 1-1 ratio in 3rd gear. No issues other than when I run it through the gears, I am shifting first to second at almost 70 MPH and rarely go any faster. It would be more fun to be shifting at a lower MPH.

[81Blackbird]

0.68 x 3.42 = 2.32 final drive ratio. 
0.81 x 3.08 = 2.49 final drive ratio.

Not a big difference on the top end.

What it the 1st gear ratio on the Tremec?  Multiply the first gear by each of the rearend ratios.  The one that comes out between 10:1 to 11:1 would be the one to go with.
As mentioned, with the torque rating you have there will not be any issues getting the car rolling.

I'm running a 3.27 first gear, nearly stock, with a 3.42.  Gives me 11.1:1  That is stock with the Doug Nash Trans.  Final drive is 1:1  65 MPH at 3000 RPM.

[Seven]

There's a gear ratio calculator on the Tremec website.

My current 65 mph cruise RPM is 2770, but would drop to 2000 RPM with the 3.08 gear/.81 OD set up or 1880 RPM with the 3.42 gear/.68 OD.

Both of the TKX versions I'm looking at have a 2.87 first gear, so the overall ratio  would be 8.83 with the 3.08 vs 9.81 with the 3.42.  With the torque output I have I don't think either one would be an issue.

There's a big difference when you kick cruise speed up to 75 MPH when the 3.08/.81 RPM hits 2600 and the 3.42/.68 combo is only turning 2175 RPM.

[81Blackbird]

The answer is clear if you do a lot of highway driving.
She would save on wear and tear and definitely gas

[nas t eh]

Seems simple, you have 3.42 gears that you can reuse if they are in good shape and you get a better total ratio in both first and OD.

[scarebird]

I had two Tremecs - the better of the 2 was this:

TCET18083

3.27

1.98

1.34

1.00

0.72

I was running a 2.93 (previously with M22Z).  Note you want a lower numerical rear as driveshaft speeds are critical.

[b_hill_86]

I had two Tremecs - the better of the 2 was this:

TCET18083

3.27

1.98

1.34

1.00

0.72

I was running a 2.93 (previously with M22Z).  Note you want a lower numerical rear as driveshaft speeds are critical.

So a 3.08 would work well with this setup?

[scarebird]

Very well.

[b_hill_86]

Awesome! Thanks for the info. I’d been worried when I put a 3.08 I mine I may have limited my possibilities of installing a 5 speed OD trans someday.