I have a 70 cutlass with a 12bolt rear end it has 4.11 gears right now and It is to much for me want to change I have th350 trans stall 2500 chevy 350 motor pro comp heads don't know what cam the guy I bought it from just said comp cam did not know part # what gears 3.73 or 3.55?

 

Unless your cam is really radical, chances are that either 3.73 or 3.55 will still get it off the line OK, so it's your choice of what you want on the highway. 3.73 is only 10% less than your present 4.11, so when you're curently spinning 3300 RPM, it will only get cut down to ~3000. 3.55 is a really nice all-around gear. But again, that's only if your cam isn't too radical. How's your idle?

Next question:
Is your "12-bolt" rear end an Olds or Chevy? If it's an Olds, this whole conversation is meaningless, as no one makes a 3.73 or 3.55 ring and pinion for it. You'll have to choose between 3.90 and 3.42, and even then you need just the right carrier inside. If the rear's a Chevy, you're golden -- you've got a gazillion ratios to choose from.

 

the car sounds choppy & the rear differential is a chevy thanks

 

If you have the smooth cover, round, 12 bolt cover it is a Oldsmobile type "O"

If you have the round cover with an inverted "V" stamped in and 12 bolts it is a chevy.

gear are more plentiful for the chevy. 3.55's are a good choice.

If it is the Olds type "O" then you need a carrier and 3.42 gears.

Gene

 

If your trans has any flaws, you might consider an O/D trans - best of both worlds!
The O/D ratios reduce the rear ratio even further for the highway, and you still have the grunt for acceleration.

 

It's a chevy rear end so you would choose 355 over 373 gears

 

YEP,
3.73 compare to 4.11, not a huge diff. Still a lot of RPM on the freeway.

Gene

 

You want RPM'S?
I've got 4:33's

 

I've had 3:55's - great around town, but didn't do a lot of highway - around 2500 RPM @ 55 - but who goes 55?

 

So took my car to auto shop and asked if there is a way to tell what gears in the rear end he said yes so he put the car on the lift and spun the tires I saw that he marked the yoke and then he said that I have 456 gears I had had 373 before in a mustang It was a 5speed my cutlass is a 3speed but I can't even get on the freeway the guy who sold the said 411gears but he new nothing about the car

 

Going from a 3:70 to a 4:11 on my '56 chev with a M-22, I only noticed a difference in 4th gear!
But when premium was 105 octane and $ .369, who cared??