I am going to pull the trigger and install one. Now I need to decide which one. Tremec makes three options. 1977 Cutlass 455, rear gear is a 2.73 open and I doubt I will be keeping it. What are you running? Are you happy or do you wish you went another route? It will be a cruiser but I am looking for out of the hole performance and maybe under 2500 RPM cruising at 75 MPH. TIA John

 

Tires will need to be replaced also thinking 255/60/15

 

3.55 or 3.73 with the 0.68 OD you'll be at 2,220 - 2,330 RPM at 75 MPH. With those tires you really don't need any deeper rear end ratio (lack of traction), so I would not even consider anything more aggressive than 3.73's.

If you don't do a ton of highway driving and want the ability to use OD a bit more around town, the 0.81 OD will be at 2,685 - 2,820 RPM at 75 MPH. But you'll be able to use the OD a bit more at lower speeds... for example with 3.55 gears and the 0.68 OD at 55 MPH you'll be pretty low at 1,680 RPM and it may feel a bit sluggish, but with the same 3.55 gears and the 0.81 OD you'd be at 2,000 RPM at 55 MPH.

https://spicerparts.com/calculators/...rpm-calculator

Is the engine stock? If it's built/modified, what was done to it?

 

Thanks for the reply! Honestly OD is not my biggest concern. I am looking for ease of install and return on investment when the time comes. 3.55 was what came to mind first. I want it to pull off the line but get through the intersection before having to shift, then save the engine while cruising. 4th gear in town is fine. Here is a link to the engine build. Jump to post #19 if you do not want to read it all. Hoping to have a part # for the cam on Monday.

https://classicoldsmobile.com/forums...needed-152771/

 

3.55 rear gears, with the TKX 2.87 1st gear, and 27" tires you'll be at 43 MPH at 5,500 RPM. You'll get through the intersection quickly, how much smoke is up to you.

 

I have the 0.68 OD, and Nick Is right, it’s a little sluggish in 5th at low speeds, with 3.73 gears.

 

The rule of thumb for a lively first gear is to have an overall ratio of about 10. You will have 3.55:1 and 2.87:1, which when multiplied gives 10.2:1 overall.

Perfect pick!

 

Thank you all. This is the information I was hoping for. I have been talking to Joe at https://dederichsmotorsports.com/ and he is awesome. Looking forward to getting it done. Next up is a drag race with the wife in her daily driver.

 

In my latest resto of my 71 442, I went with the TKX and love it. I also have 4.10 posi gears and would not change them for anything lower. It rips out of the hole quickly with 575 HP and runs 3K RPM in 4th and 2200 RPM at 75 MPH in 5th gear.

Tires are where it's at for good traction and less smoking. I have really good and sticky tires.

 

What are you running?Part Number

Style

Torque Rating

Input Spline

Gear Ratios

1st

2nd

3rd

4th

5th

R

TCET18083

GM

600 lb-ft

26

3.27

1.98

1.34

1.00

0.72

3.00

TCET17722

GM

600 lb-ft

26

2.87

1.89

1.28

1.00

0.81

2.56

TCET17805

GM

600 lb-ft

26

2.87

1.89

1.28

1.00

0.68

2.56

 

TCET17722

https://dederichsmotorsports.com/tre...ansmission-gm/

 

I bought the last two Tremecs from Modern Driveline as a kit. Here are the specs from my order for the 442 restoration:

Part #: MD-501-17805
GM TKX 5 Speed
Gear Ratios: 2.87, 1.89, 1.28, 1, .68
Rated torque: 600 ft lbs.
BOP Bellhousing
Hydraulic Throwout Bearing
GM Kevlar Clutch


My rear tires are Mickey Thompson Sportsman
Front Rims: 15x8 Billet Specialties Win Lite
Front Tires: Mickey Thompson Sportsman 27x8x15

Rear Rims: 15x10 Billet Specialties Win Lite
Rear Tires: 28x10x15 Mickey Thompson Sportsman