Ok its simple . I have a 455 from a 68 Toronado in my 72 Cutlass Supreme . I bought a built TH350 transmission from a local speed shop and installed that also . Now of course I need a posi rear end . I'd prefer to get the entire rear axle used locally if possible . I never messed with the internals on a rear end before and realize you need to use a dial guage and shims to do this correctly ( I don't want to mess it up ). I drive on the freeway so I want to know what gear ratio would be good , whether I should stick with the 8.5 , and if someone here has one they'd like to part with . Thanks

 

Your tire height makes a difference on gear choice. With the stock tire height, 225/70R14 with a 2.73 gear, you are around 2300 rpm at 60 mph. I say a 3.08, that puts you around 2600 rpm at 60 mph. With something like a 275/60R15 tire, it will put you around 2500 rpm.

 

I currently have P225 75 R15 on the car . I bought SSII 15" rims from someone on here a few years ago

 

I just want to be able to drive on freeway and sometimes bury the needle . The car originally was a 350 4 barrel with single exhaust with a one wheel wonder in the back . I don't know what gear ratio that was but the VIN number had a " K " in it .

 

The standard ratio with 350 and AT would have been 2.73, but 3.08 and 3.23 were available options.

 

Ive got a 342 in the rear its about 3200 rpm at 70 mph w 255x60x15s. If you drive alot on the freeway id say 308 of 323.

ive got the original 10 bolt w a posi and a 455

 

3200 at 70 is nothing. People are spoiled by OD transmissions. My 62 runs at 3000-3500 RPM on the freeway with no issues. Been doing that for ten years now, including Power Tour last summer. 3.08s will disappoint.

 

I have 3:08's with a locker, they are fine for what I do with the car I have enough power the shred the tires at will.

 

Thanks for all the answers . Now I just have to find a axle to buy

 

Im pretty sure you can buy the posi and gears and install in your existing rear end. I had mine built by a guy who knew what he was doing.

Re joe and 3200 rpms.

agreed its not like im at redline but if he drives the freeway alot a lower rpm provides less engine noise and possibly more interior comfort.

 

Any Coporate 10 bolt 8.5 gears will fit your factory housing, many available brands and ratios. Plus you have the superior bolt in axles. Get a set of 3.08 or 3.23 gears and a Trutrac posi with the install bearing kit and you are good. Installing gears are not hard with the right tools and some know how.

 

Sweet, at least 275 HP and 375 LB FT net stock. Your adding 100 HP and 80 LB FT over stock doing nothing....

Nice, I'm assuming similar strength to a TH400 without as much power consumption. Plus a shift kit...

Good idea, old school car guys will usually discourage this mod from gearheads hands. Always heard properly setting a rear is akin to artistry.

Very tall tire, over 1 inch taller than stock. So if your Supreme is 2.73 rear your tire is effectively turning it into a 2.62. If you go 3.42 and decide to keep current tire height it will effectively be 3.28

Corporate 8.5 is great. Strong without excessive power consumption. I'm leaning with 3.42 ...

So many positives, car should haul *** !