Rear end help needed please...

[Rich72C10]

Nice looking truck 1G1YY!

This post interests me.... I suppose I didn't know about the 3.42 gears.

So I have open 3.08 (or is it 3.07?) in my 72 350/TH350 setup. I have toyed with the idea of the various options to go OD, which I know my current rear end won't be so good for OD. But I'm not 100% convinced I want to go OD (ROI on that cost). My tires are 275/60/15 on Rally Rims.

So it comes down to what I want to do and keeping in mind the "ifs" - if I do or if I don't do OD. Or do I leave things completely alone, as I keep telling myself. I even thought about just going positraction with my current gears but that seems silly to me - if going that far might as well do the gears too.

I have a feeling I wouldn't be happy with 3.73 on the highway if I never did OD but perhaps I'd be happy with 3.42 positraction either way (OD or not)? Heck everything around me in Texas is 65+ and if you are doing just the speed limited you are pissing people off LOL

Oh my question: Anyone running 3.42 positraction with a setup similar to mine? What is it like, street level, highway, MPG?

[MikeB]

Originally Posted by Rich72C10

Nice looking truck 1G1YY!

Anyone running 3.42 positraction with a setup similar to mine? What is it like, street level, highway, MPG?

I ran a 3.42 in a 55 Chevy car with 27.5" tall tires and Muncie trans with 1:1 fourth gear. Your tires are nominally 28" tall which is only around 2% taller.

Have to say the car was very busy on the highway at anything above 60-65mph. And that was without torque converter slip of 200-300 RPM or more, which you'll have to take into consideration. And I'd forget about 3.73 gears, unless the truck will seldom see sustained high speeds, or you plan to go with an OD trans.

You are right about OD not being a good fit for 3.08 gears. A buddy of mine has a 37 Chevy with a mild 350 and 700R4. And even with 3.36 gears, he normally keeps the trans in 3rd gear until around 50mph. And converter lockup is controlled with a switch that he doesn't engage until 60mph or more.

Looks like the factory did a lot of 700R4s with 3.42 gears, because it was all about fuel mileage, not drivability.

[1G1YY]

Rich, You are correct. It is a 3.07 rear not 3.08...my bad.
And thanks...my truck is still a work in progress but getting close.
My tip is that you use an engine rpm calculator...it will be your friend.
I just did this with your setup....3.07 rear, 28" wheels at 65mph .
With a 3 speed auto your engine will be doing 2459rpm.
T700 4sp auto the engine is spinning at 1721rpm....much nicer, but at a cost in time, effort and bux to install a T700.
The economical way of achieving lower rpm's at highway speed would be to fit taller tyres.
I would not worry about a posi as that will not affect engine speed, just traction to both wheels esp at take off.
Just some food for thought...

[MikeB]

Originally Posted by 1G1YY

My tip is that you use an engine rpm calculator...it will be your friend.
I just did this with your setup....3.07 rear, 28" wheels at 65mph .
With a 3 speed auto your engine will be doing 2459rpm.

Don't forget torque converter slip.