OldOldsLover

Hi,
For my 1972 Cutlass Supreme...
Around 1985 I rebuilt a 1970 455, E heads, approx 9.3:1 compression, I don't remember the cam specs but it's for power 2,500 to 6,500 rpm, single plane Torker manifold, I retarded the cam timing 4 degrees, which I think just shifts the peak torque up another 200 rpm. Transmission is M20 Wide Ratio.
My current gears are 3.23, but slightly smaller tires so its effectly a 3.33.. The differential is a GM corporate 8.5 10 bolt with a S clip positraction carrier.

I am going to change the ratio.
I don't do much highway and I don't remember thinking the engine was screaming when I did 65 or 75 with the current ratio.
I really liked the power in the upper rpms and my engines spins fine up there.

I will most likely have tires close to the stock diameter about 26"
ideally i will upgrade to a tremec 5 or 6 speed like the tkx in the future if/when my financials allow. But that will primarily lower my highway rpms. It will effect 1st gear also but to a lesser degree.

My question is....
I'm thinking of going with a 4.10, or 3.90.
Got any real world experience wih 4speed big block with 4.10 or 3.90 or 3.73?

I made an excel spreadsheet that shows the rpm and mph in each gear. I don't see anything that looks extremely different even with 4.10.
I am looking to have it for fun and quickness not highway top speed.
Thanks
OldOldsLover

OLDSter Ralph

What body are you using ? Your biggest problem could be traction, you definitely want a limited slip differential. I like 3.90 for mix of city and highway. 4.33 is good for get up and go.

OldOldsLover

I undate post...
1972 Cutlass Supreme
I have a gm corporate 8.5 10bolt Posi Diff now.
I plan on upgrading to 17" wheels so hopefully they'll get better traction with better tires.

maybe if you could expand on how 3.90 are for a mix,,,and 4.33 are for get up and go..
is this your personal experience?
Thanks man

VI Cutty

You need to know your tire diameter to get the right information. I'm going through a similar decision process myself, except for a 5 speed, but I was talking to a friend yesterday who has a Pontiac 400ci putting out some big power with a Muncie 4 speed. He's running a 3.90 rear gear and 255/70R15 tires which have a 29" diameter. His tire selection was aimed to try and tame the 3.90 gearing a bit.

Even with that setup, he can break out the rear wheels in 2nd and 3rd easily enough.

OldOldsLover

from the information ive been gathering i think the tires will be close to stock diameter..about 26".
I understand the relationship of tires on gears, as well as on braking.
I don't think i want to go with a tire thats that much taller than stock but I can't say for sure.
Thanks

can you tell me what you mean by "tame"?
is it that the tires lose traction and spin up?
or is it something else?

My primary concern is frequently of shifting.
i think that i can control the tires breaking lose by self control on the gas pedal.

thanks

Andy

390 is an all around fun gear as your goal is acceleration around town, stop light to stop light per se, I wouldn’t over think it. Must be a stout cam to have that power curve then add the torker and your power band is on the upper end.. 390, 410 either will put a solid grin on your face

VC455

I did the MS Excel routine too and spent a bunch to get a 4.10 into my Type O. I found I could have gone to 4.33 because I have 2370 pounds over 28" tires that don't spin unless I launch above 2000 rpm.

And, yes, I think any ratio in that area will not be too much different. Next time I wouldn't overthink like I did.

442Dude

Good morning! I just finished a resto on my 1971 Olds 442 and have the Tremec TKX and 4.10 posi gearing. My rear tires are 28: diameter and fit well and look good in the wheel wells with the car's stance. I'm quite happy with the combo. The tires I have stick well to the pavement and provide the traction I need to enjoy the gearing and have fun on the street. My engine RPMs are significantly reduced with the 5th gear if I want to drive on the high way or at higher speeds.

I would not change the gearing at all and have similar tires and the same gearing my BBC Chevelle as well.

Allot of your choices for the available combos have to do with your preferences......good luck!

OldOldsLover

Great advice
Hard to apply
Sometimes I analyze till i paralyze.


Thanks for all the replies.
I ordered Motive 4.10 and master rebuild kit
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