I was looking to get more oomph out of my 70 Cutlass S using a 3:91 posi i found on craigslist. The car is entirely stock now, but a 350-375 hp rebuild of the small block is in the works. I've read on a forum recently I posted regarding my heads, that i may have too much gear for my application. I dont see any harm in the gearing other than gas, but I really couldnt care about that. The car is a weekend cruiser and I'd run it on children's tears and baby seals blood if i had to! I've got 275x60x15" tires out back which I think would be a 29" tall tire. What kind of 60 MPH RPM's would i be running, and more importantly, is this too much gear? Thanks for any help!!

 

Here is a good calculator that you can adjust ratio and tire sizes to figure out different scenarios. Your rear tires are 27.99 in diameter.


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


Tire calculator:


http://www.discounttire.com/dtcs/infoTireMath.do

 

Hey thanks alot! That calculation works super slick. According to the calculations, I'd be running just under 55MPH at 2500 RPM. I can definitely live with that! Plans are for a 2004r tranny at some point in the future which would probably put me back to where i am now. Ok, having said that, is there a diffence in choosing a cam between the turbo 350 I have now, and the 2004r I plan on installing one day? Tourqe converters work the same for either tranny?

 

The gears are not that different in the trannys to affect the cam. Pick the cam based on the rear gears and engine specs. I'm pretty sure the torque converters are different for the TH350 and the 2004R if that's what you are asking.

 

Your 3.91 works out to really be a 3.63 gear with the 28" tall tire. Perfect street gear. I installed a 3.50 gear with 28" tires and it was a pig getting it moving with a standard shift.

 

What I think you meant to say was that 3.91 gears with 28" tall tires is equivalent to 3.63 gears with 26" tall tires, which would be correct, however the O.P. never mentioned a different rear tire size.

 

I get 2600 rpm with your wheel and ratio info...??? Now since most freeways are 70 mph 2833 at 70 MPH. You also need to remember about torque converter slippage in an Automatic trans too, which will increase RPM on top of this number. If your car is stick shift / Manual trans then slippage is not as much of an issue.

The camshaft selection will also determine the RPM your engine should run at also... If you have a BIG overlap cam your engine will run more correctly at a higher RPM as that is where the cars power band will be.