posi rear end

[scarborough]

i have a chance to purchase a posi rear end with a 373 gear i was told it out of elco would like to run a 308, 323 or 342 gear. it a bb Olds with a th350 transmission and 255/60/15 on the back. its a daily drive i would like a gear i can drive on the highway but still take advantage of the bb torque . my question is do i need to change the pinion also if i went with a different gear ratio or can i use the same pinion and just change the ring gear

 

[DRIVEN]

Need to be replaced as a pair. BBO will have no shortage of torque. Play w>th a gear calculator to see what makes sense. I'm thinking 3.08s for regular highway usage.

 

[scarborough]

Need to be replaced as a pair. BBO will have no shortage of torque. Play w>th a gear calculator to see what makes sense. I'm thinking 3.08s for regular highway usage.

thanks for the feedback.

 

[olds307 and 403]

As said, the crown and pinion have to be a matching pair. I went with a solid pinion spacer. I matched the original 2.56 pinion depth when I installed 3.42 gears and backlash must be in spec. Here is a good calculator, add a 1 in place of the .67 and add 200 to 300 rpm for torque converter slippage. http://tech.oldsgmail.com/axle_RPM.php
You will be around 2500 to 2600 rpm at 60 mph with a 3.08 gear and your tire height.

 

[69hurstolds]

You won't feel much difference between the 308 or 323s. So either one of those should be fine. Note that many "standard" gear sets for Olds 442s with 455 were or around 3.23.

The 3.42 on the other hand...if you do some drag racing occasionally, then this is probably one of the optimum gear sets with the 3 speed transmission you have. Otherwise, for mainly highway, stay away from this one.

If you're not handy with setting gears and shimming, then farm it out to a trusted mechanic. It's not hard, it's just more math, a few special tools (to do it right), and PATIENCE plus have a couple extra crush sleeves on hand just in case. You may be checking/pulling/installing the center section a few times to get it right where it needs to be. And remember, the ring gear bolts are reverse threaded than what you're used to (righty loosey, lefty tighty).

Whichever you choose, make sure to match up your speedo gears to correct your mph as needed.

 

[Rt Jam]

It really depends on what YOU consider a comfortable highway RPM is. With a TH350 3 speed trans 1:1 high gear.
Any choice will be a compromise on the other end. Overdrive is the answer.

 

[69hurstolds]

It really depends on what YOU consider a comfortable highway RPM is. With a TH350 3 speed trans 1:1 high gear.
Any choice will be a compromise on the other end. Overdrive is the answer.

Not discounting what you're saying, because in a world without bounds, it's sound advice. But here, it depends on the question.

Spending more money on an OD transmission is sort of outside the realm of the OP's discussion limits here, although if an OD transmission change was in his future and we're spending his money, I'd say toss in a BUILT 200-4R (.67 OD- keep in mind a bone stock one will likely be torn up in short order) or a built 700-R4 (.70 OD).

With a 2.50-ish effective final drive ratio depending on which transmission used, this would still be better than the 3.08 for highway, and leave the 3.73 in there. Best of both worlds. Tear up tires street light to street light while being decent rpm on the highway.

This is, of course, only applicable if I had access to the OP's wallet and funds.

In the bounds given between 3.08, 3.23, and 3.42 with a TH350, then I still recommend one of the first 2. MUCH cheaper than going with a new transmission, even if you replaced every bearing in the unit. The cost of the ring/pinion will likely be roughly the same regardless. A bit more if it's a 7.5 rear end, though. Complete rear end master kits with gears/bearings, shims, bolts and seals, etc., are typically $350-375