My Cutlass is currently running a rear end that includes non-posi Hi-way gears. How many RPMs should I expect at 70 mph if I switch to a 3.42 gear? It is of course a 1.1 ratio coming off of third with its turbo 350 trans and I'm sure a few of you know what I should expect. - Steve

 

My tires diameter are very close to factory spec.

 

this will help...

http://www.angelfire.com/fl/procrastination/rear.html

 

Thank you, this is very usefull!

 

Ouch. This certainly looks better with the Overdrive 200R ratio rather than 1.1.

 

I was turning 3200-3300 rpm @ 70 mph with 26.2" dia. tires, 350 trans. and 350 engine.

 

Put a 27" (255 60 15) tall tire on it & 3.23 gears. That combination will give you a great compromise between performance & cruise rpm. You will need to drive your car a long ways & often to recoup the cost of the OD trans.

 

Or 3.42 with 275/60/15 28" tall tires. I agree on the cost savings of an overdrive including labor if you do not do it yourself will take a long time to pay for.

 

I don't recall the overdrive as being too darn dependable in a Caprice, maybe better in a Grand National though. I'm sure the $2500 - $5,000 units are pretty well built, but not worth the money to me. My tires are brand new Eagle F1's at 25.75" and are staying. They are 225/55 16's and I pleased with how the 16" wheels look too.
I have the opportunity to buy a nice 1970 3.42 posi and I'm leaning that way even if it does push my RPM's over 3000 to drive 70mph. I'll sure burn some gas on the Highway though.
Thank you for the input guys, and thanks again for the link to the calculator Qwik71442!