I am looking for opinions on what rear ratios would be reasonable for a overdrive setup. When I do my build this winter my thoughts were to swap in a built 200r4 behind a stock/mild 455 in the 70 Cutlass. It currently has the original 2.56 Type O rear in it. My initial thoughts where to swap in one of the aftermarket carriers with 3.42 gears. I have recently learned that the 3.42 Richmond gears are not currently being made/available. So I need to look at alternatives including finding an original rebuilt unit or one to rebuild. I plan to use the car mainly for a fun weekend car that will see a fair amount of highway trips along with the cruise night fun. So I am looking for a combo that gives good performance while retaining highway drive ability. I am running 245 60 on 15" wheels. So playing with the ratio calculations in overdrive, I determined 3.42 put it slightly less rpm than the 2.56. The 455 gives me some flexibility with the low end torque.

So after a long story, my questions are: What ratios have people had success with in similar setups that I can consider when looking for a unit to use. Is 3.23/3.08 to tall to consider? 3.90 will end up a little lower than the original. All improve the take off over what I have, so I gain no matter what there.

Thoughts and suggestions appreciated.

Jeff

 

I just had my kid's (red) car on highway yesterday morning. 350 w/ 3.08, 26.5" tire turns 1700 @ 70, 2000 @ 80 w/ converter locked up. If rpm gets to around 1500 it is slightly "luggy" but we're talking maybe a 300-325 hp engine, a 455 would still be happy. It is very pleasant to drive, just disable the lockup (hit switch or tap brake pedal) if it goes below that. Put appx 50 miles on it between secondary highways & interstate, gas gauge moved from "full" to bottom edge of full line. Blue car has 3.90 w/ slightly more diameter (15", can't remember size but short end of things) & spins appx 2300 @ 70. Also very pleasant to drive though if you even flex a toe hair without realizing it the pace has increased 15 mph. Will tolerate 50 mph but is not really happy below 55. Could drive @ 85-90 all day comfortably but I don't think the mpg would hold up very well!

I think 3.42 would be a very nice compromise between acceleration & highway cruising w/ a mild 455 but you could go a bit either way & not regret it.

 

Math is your friend. Fourth gear in the 200-4R is 0.67:1. Simply divide 2.56 by 0.67 and you find that 3.82:1 gives you the same top gear RPMs that you have now. Since 3.82 gears don't exist, go with 3.73s or 3.91s and you'll be very happy. Off the line acceleration will be markedly improved and you'll have the same gas mileage that you have now...

...WHEN DRIVEN THE SAME!

 

You just can't get a 3:73 with the O-axle,but you can with the 71-72 8.5" 10-bolt,or the 12-bolt Chevy.You can also get a 3:55 with the 12-bolt Chevy,but not the O-axle,or the 71-72 8.5" 10-bolt.

 

I just put 3.42s in mine, my 455 is much happier now. A 2004R has slightly higher numeric 1st and 2nd gears than my TH350. I think 3.73 or 3.91 would be good too, but I think 3.08 or 3.23 would be unnecessarily conservative with an overdrive.

 

Just replaced my stock rear with Chevy 12 bolt 3:55 in my 1970 Cutlass. Daddy happy.

 

3.42 is a nice gear. It's my favortive gear actually. I had 3.42 in my old Delta with a th400 tranny. It was below 3000 on the highway @ 70, but I always drove that car slower than my Alero. If I had 200R4 in it, I beileve it would have been 2300 @ 70

3.73 is a pretty decent gear, but I wouldn't want that without overdrive

 

I have a mildly built 455 in my Vista with a built 200 4 R & a 4:10 gear. It gets off the line really well, but a big heavy car like mine needs that. Cruising along at 70-80 is no problem & I still get about 12-13 mpg at those speeds. I have 255/60/15 tires. Really is the best of both worlds with the overdrive & the 4:10's.

 

Here's a thread from another member. He went with 3.90's and the 200R4 in his 1965 Cutlass.


https://classicoldsmobile.com/forums...new-combo.html


John

 

put a 9" in it, I built one with a friend for my 69 cutlass, it wasnt that hard, then your gear options are almost limitless

 

YAH, BUT THEN YOU HAVE THAT WEAK A-- 9" IN THERE. LOL.

AND A FRAUD (can not say the F word) REAR IN YOUR GM CAR.

UNLESS THAT CENTER SECTION IS A PRO THIRD MEMBER FORGET IT...

Please ignore the caps...lol Not yelling just had the caps on, sorry...

Jim-J D

 

Ford, aka "Dearborn," was a drivetrain vendor to Olds, via the HD 3 speed top loader transmission. Just as my chassis service manual discusses the Type C, Type O, etc. the 9" is a Type F ten bolt, so there! lol.

 

Hey Dont get me wrong, I love the O type as much as the next guy, but before I dish out the kind of money it takes to build one, even building a 12 bolt is expensive, ill build a 9" Its true, the base model carriers are a little on the weak side, I broke the one in my truck, but the one behind the 455 in the olds held up fine between when it was built, and when I sold it.

That being said though, for anywhere from 150, to 700, you can get a nice nodular 3rd member that will easily hold up to any amount of a beating a 455 will throw at it.

Of course if your running slicks and a 4 speed, you might need an axle tube brace of some sort - still cheaper than an O type!

 

I have yet to actually need a 9". I have run the O-axle well into the 11's,and the 8.5" 10-bolt & 12-bolt Chevy into the 9's. Stock parts? NO,but the 9" wouldn't have stock parts either. I think he is trying to stick with the 12-bolt O-axle for appearance & originality.It sounds like he just needs to pick a ratio,not a rear.

Let's not forget the DANA 60,where you will always have 2 full teeth engaged at all times.

 

God - I love feeding the fish...lol

Jim