7D2455

My '72 currently has the 8.5" 10 bolt open differential with 2.73 gears. Its got a 455 mildly built, a TH400, and I'm running 27" tires in the back. The car never sees the highway, only driven around town for fun.

I just picked up another 10 bolt that has factory posi and 3.23's, as well as an extra set of 3.73's. Do you guys think I should swap in the 3.23 axle or would it be a better choice to pay the money to have the 3.73's setup with the posi?

Indy_68_S

My math comes out to about..

3.23 = 2800 RPM @ 70 mph
3.73 = 3250 RPM @ 70 mph

....the 3.73 would definitely be more fun around town.....

Lee_A

Can you spin the tires already?

Steeper gears will make your car quicker off the line ONLY if you have traction.

3.73's are just too much. 3.23's are a great gear for most 455's. But if you can light up the 2.73's already, you won't gain anything.

3.23's and go MT drag radials would be fun.

Lee

7D2455

I can spin the tires pretty easily, but the tires will still grab. It just depends on how I drive it. I dont know a whole lot about how certain gear ratios will effect a vehicle. I know that the higher numbers are supposed to make more torque, but if you go too far then all you will do is burnouts. So I figured I would ask the experienced guys...

oldzy

If I had a torque monster like the 455 and those 2 gear choices... 3.23's easily. Now if I had a 350 (like I do) then it would be the 3.73's.

Lee_A

The 3.23 posi will be better than a 2.73 open.

3.73's will be too much.

Lee

7D2455

I used to think that 3.73's were always the best, and 4.11's were even better, but I guess that's because thats what everyone around me talked about while growing up as a kid, and thats probably because they all had small block chevys haha. Im glad I could get some input from you guys. I think I might be going with the 3.23's, especially since I can just bolt that axle in and run it with no extra costs.

507OLDS

The 3:73 is a popular gear for a street/strip 455.If you do not do any highway driving,I would go with those.They will help get the most out of your engine performance.You will not need any more gear than that though.If your car has AC,then you might want to consider the 3:23's.You could freshen-up the 3:23 rear,try that,and have the 3:73's installed into your existing rear,along with a posi unit.

455man

Why would A/C affect your gear choice?

Indy_68_S

AFAIK, its so the higher engine RPMs don't over speed the compressor...

Rickman48

As a youngster, I also thought 4:11's were better than 3:70's, and when the 3:70's went away, got a 4:11! [yes, early sbc]
Only noticed a difference in high gear, though, with a M-22.

With the torque of the 455, I'd also also choose the 3:23's - 3:73's are a little too much - now 3:42's would be perfect!

455man

That was what I was thinking. Thanks.

507OLDS

Those cars that came with AC were offered ratios up to 3:42.If a ratio higher than 3:42 was ordered,there were certain warranties voided.
The higher rpms can actually disengage the compressor clutch,so you would not be able to get AC.

7D2455

Well luckily mine no longer has a/c.

chrisneu68olds

@ Indy_68_S
What formula did you use to calculate this?

BlackGold

I'm still convinced that the factory limitation of 3.23 for cars with A/C had more to do with engine cooling than over-spinning the compressor.

Cars with A/C received a heavier-duty cooling system.
Cars with low gears (higher numerically) received a heavier-duty cooling system.

It's simple extrapolation to say that, if either of those options alone warranted the heaviest-duty cooling system offered, then both of them together would require the superdupergonzomotherofall cooling systems.

Of course, the factory didn't know if you live in Death Valley or the Yukon, so in the interest of reducing their warranty costs, they played it safe. Your mileage may vary.

Indy_68_S

I have a little old standalone app the lets me put in tire size, trans type/ratios, diff ratio, MPH, RPM,etc. You just leave one field blank and it fills it in.

But, there are numerous ones out thar on dat IntarWeb.....

Matter of fact, here's an Olds related one now !

http://tech.oldsgmail.com/axle_RPM.php

507OLDS

Blackgold,
Yes,that had something to do with it as well.

7D2455

So the 3.23 axle a bought turned out to not be a direct bolt in with my Cutlass Supreme, so I took the gears and posi carrier out of it. In a few weeks I'm going to have the posi put into my current 10 bolt rear end, and I think I'll have them put the 3.73's in rather than the 3.23's.

oldcutlass

What brand tires do you run - imma gonna invest in their stock. Lol, great choice if you don't do hwy driving. Are you running a stock torque converter???

507OLDS

7D2455,
Was the rear out of a different era of car,like a 73-77?
As long as it is an 8.5" 10-bolt,then you can put the guts into your existing housing.

455man

But it has to be the 28 splines to fit the axles right?

507OLDS

Yes,any 8.5" 10-bolt from 1971 to 1988,has 28-spline axles.

Run to Rund

My Cummins diesel pickup "deserves" the 3.73 ratio it has. Oldsmobiles "deserve" 4.10 at a minimum, lol. Since it isn't a high speed highway cruiser, put some gear into it

455man

Except a corvette rearend. Are there any other 8.5 10 bolts that dont have 28 splines?

7D2455

It is a stock torque converter and fully stock turbo 400

7D2455

Yes, it was out of a '73-up full size buick, and when I bought it I didnt catch that the brackets on it were different. But it is indeed an 8.5" 10 bolt.

I would just transfer everything over to the 10 bolt in my Cutlass, but since I'm going through all this work now I want the 3.73's in it, especially since that's what I wanted in the first place. So rather than attempting to setup the posi and 3.73's in my 10 bolt I will just pay a shop to do it to ensure it is done right.

jonrare

I have a 455 with 3.73 gears with 28.5 inch tires and its great on the highway and responsive off the highway

i had 2.73 and its a big difference

My442

I run 3.91s

Not fun on the highway, but really fun around town.....

56oldssuper88

If it were mine I would run 3.73's just for the fun. But 3.23's= lower rpm and better highway cruising. The answers that were figured in that formula mentioned sound about right for that trans and those gears combo.

oldsonharmont

I'm not sure if they'll light up the tires or not, but the 3:08's are supposed to be a nice all around gear set. With your big block they're still liveable on the highway, and you'll get off the line a bit quicker around town.

joesw31

Take that rear and harvest the internals, and find a used GM 3.42 ring and pinion. Again, I would use a 3.42 ratio as that gear would give you great perfomance and streetability.

Bunser

Agreed, but I think I'll try and find a 3:42 ring and pinion to make it bearable on the highway.

7D2455

Well at this point I'm not worried about highway driving. If it does see the highway it wont be very far. Pretty much all driving I do with it is in town. I wanna go with the 3.73's, and then in the future its gonna get a 5 speed.

BLUZIN

olds 307 and 403

What trans are you planning on?

Andy

3:23’s buy a 2800 stall coverter, car will roast the tire’s or tire.. thats the best all around ratio for you..