Best Rear Gearing for an OD Transmission?

[jonstringer]

In my 1976 Chevy Sportvan SWB, I have a 350ci mated to a 700r4 OD trans to a 3.73 Posi Rear. This combination works nicely on my van, but I need your opinions for my olds.

I plan on swapping my th350 with a 2004r OD transmission. My cutlass S has a 455ci, so I'm trying to mate it to the optimal gearing. I was thinking of going with 3.73s again, but I was told that would be too high. I have trouble getting traction as it is with a 2.73 rear. So I'm thinking a 2004r and a 3.42 posi should behave great and leave me at around 2100 rpms at 70mph. Is my math correct or even close? I want to utilize the 455s low end torque which rocks at highway speeds. I'm open to your opinions and advice. What rear end gear should be mated to a 2004r and why?

[joe_padavano]

Originally Posted by jonstringer

In my 1976 Chevy Sportvan SWB, I have a 350ci mated to a 700r4 OD trans to a 3.73 Posi Rear. This combination works nicely on my van, but I need your opinions for my olds.

I plan on swapping my th350 with a 2004r OD transmission. My cutlass S has a 455ci, so I'm trying to mate it to the optimal gearing. I was thinking of going with 3.73s again, but I was told that would be too high. I have trouble getting traction as it is with a 2.73 rear. So I'm thinking a 2004r and a 3.42 posi should behave great and leave me at around 2100 rpms at 70mph. Is my math correct or even close? I want to utilize the 455s low end torque which rocks at highway speeds. I'm open to your opinions and advice. What rear end gear should be mated to a 2004r and why?

Unfortunately your math can't predict the RPM at speed unless you tell us the tire diameter also.

[Redog]

This is true.

I put 3.42's in my rear with a th400 tranny and it turns, as least it should, at 3500 RPMs at 80 miles per hour. I'd have to get up over 120 to redline, and that hasn't happened and probably never will since that's a little too high in the 1/4 mile

Anyway, play with this website

http://www.race-cars.net/calculators/et_calculator.html

[Rallye469]

3.90's
Then work on your traction problem.

[Olds64]

I think you should use a 4.88 spool.

[Rallye469]

I think you should use a 4.88 spool.

like your thinkin...

My buddy has an 86 vette (421 sbc)
He runs 4.10's with his 700R4...
The bastard cruises at 80mph all the way to the track(96 miles)
then lays down 11.1@122mph with 1.5 60 fts

Needless to say...I kinda hate him.

[68conv455]

I'm guilty of telling you the 3.73 was too high. Forget about that. Here's what you need to know:
When comparing a TH350 to a TH2004R, the 350 has a 2.52 first gear and the TH2004r has a 2.74 first gear. Therefore, the OD tranny is roughly 8% different in first gear. So, if you're converting from a TH350 with a 3.73 to a TH2004r with a 3.73, your new set-up will feel like a 4.03 rear in first gear.
When comparing a TH400 to a TH2004R, the 400 has a 2.48 first gear and the TH2004r has a 2.74 first gear. Therefore, the OD tranny is roughly 10% different in first gear. So, if you're converting from a TH400 with a 3.73 to a TH2004r with a 3.73, your new set-up will feel like a 4.10 rear in first gear. Great for effortless smokey burnouts with a 455, bad if you ever get caught in the rain.
Whatever you choose, the O.D will be a sweet .674:1

[Redog]

If I had an OD tranny I'd go with 3.73 or 3.90

Probably wouldn't go with anything in the 4's or higher if the car will be street driven.

I know somebody that had a SBC 350, TH350, and 4.10's and to go 100 miles took him 20 gallons of gas

[88 coupe]

Originally Posted by jonstringer

........ What rear end gear should be mated to a 2004r ........

Not possible to answer without knowing more about your combination, and how you intend to use it.

Originally Posted by joe_padavano

Unfortunately your math can't predict the RPM at speed unless you tell us the tire diameter also.

The question is a good indication that he wants to give you a serious answer.

Norm

[jonstringer]

Sorry it took so long to get back to this post...

My tires size is 245/60/14 BFGoodrich Radial T/A

So imagine a 455, 2004r, that tire size, and ? rear... What would work best?

[Olds64]

Make sure that 2004R is stout before you drive it. A 455 will tear a stock 2004R to pieces.

[jonstringer]

Oh, believe me, I will. I watched a guy blow up two 700r4's with his BBC 454 in a matter of a couple of months. They rebuilt it under warranty once, then denied him the 2nd time. I have a great tranny guy in San Antonio, TX. $900 for a 2004r that's basically bullet proof. He doesn't really charge me for labor.