Planning to change rear end gears, I’d like posi, and maybe 3.08 or a little more! 3.43? What would be the way to go for this? Have the original 2.74 10 bolt I believe.

 

depends on your end goal and the extent of any other performance mods you might be contemplating.

Just a basic gear swap for street use, either 3.08 or 3.23 will improve your off-line acceleration. 3.23 with a TH350 trans will bump your highway speed RPMs up a noticeable bit though.

I have a 3.08 posi in my '72 which replaced the original 2.73 open set it came with - nice overall improvement but acceleration grunt isn't as pronounced off-line as the 3.23 posi set I had in a prior '72 Skylark ragtop.

The 3.23 set could break the rear tires loose from a stop with little effort but I didn't like the high RPMs it ran the motor up to at 70+ mph.

 

I should have given more info! Mainly would like posi, It’s a 455 car with 400 trans, I’m currently building a 455, should be in 450hp range. In time hopefully a 200r4 to bring revs down on the highway.

 

Still running stock 455 while building the other. May not have it done this summer.

 

IMO not worth ditching the TH400 just for another gear.
Comparing a TH200 to TH400 is like comparing a 1971 Eldorado to a smart car.
A 200 will not survive (long) behind a 455 w/o significant upgrades.
Spend that money on a Gear Vendor OD. Or a modern version of the TH400, 4L80E, with its stand alone TCU.

 

Good idea, what way to get to a 3.08 or bigger?

 

A 455 will run all day long at 3500 RPM on the freeway. You don't need an overdrive trans. If you must have one, go with the 4L80E, which is effectively a TH400 with an extra overdrive planetary gear set and clutch pack between the pump and the rest of the trans. And the difference between 3.23 and 3.42 ratios is a whole 6%.

 

som times the price on a 3.23 can be more than a 3.42 which is more common too. And if your doing an overdrive the 3.42 for the MC will be better than the 3.23 IMO I have 3.55 and 700r4 in the SS 454 monte and it is just about perfect - great highway and 3.06 first with the 3.55 is grunt as all. I do not pick big cams so really any ratio will sizzle them tires with my motor ran 12.90 with the 3.55 so that is plenty good for me ... you can not get traction on the street anyway.

 

Before wanting to swap out, can go budget and get 30 spline axles, albeit a C clip rear, and use a 77-96 B body axle. 89-92 sedans with towing package typically had a 3.23 or 3.42 axle. 93-96 all had pretty much 2.93 or 3.08. A rare 3.23. Use a SVL-2022648 axle to get your bolt pattern back and use drums from an 89-91 police package or taxi.

While you're in there, you can use the 77-96 B body spindles to get a 12" rotor also and use the 89-92 F body with performance package to keep your bolt pattern. I have 2" Impala drop spindles on my '77 and an Impala disc brake rear, swapped in a 3.42 and TruTrac.

 

marketplace. I just picked up a complete 3.23 posi rear last year. I suspect it's from a 73 or 74 Grand Am originally.