Diff ratios and posi swap?

[DmcL]

Ok first I want to confirm what came stock in the 85 Monte Carlo SS. Was it a 3.73? Secondly my tach doesn't read correctly so what sort of rpm would I be spinning in 4th (200-4R) at 60mph and at 80mph?

Wanting to know engine speeds as I might want a nicer ratio for get up and go but I don't want to rev sky high at 60 or 80.

Lastly its a 10 bolt rear end in the SS isn't it? I haven't counted but I assume it's a no brainer and 10 bolt refers to bolts on the rear cover. Assuming that's the case what other cars could I take a posi from to put in my SS because 1 wheeler diffs suck big ones. If I got a posi how would I go about changing the ratio? Is there a kit I could buy?

 

[RustRocket]

Don't take this information to heart, as I am no expert. But I believe if it's a good 'ol GM 7.5" then you can swap in the 3.42 or 3.73 Positraction gears from a Third-gen IROC Camaro. I'm not all that knowledgeable, so I would definitely get an expert opinion(mechanic or something like that).

 

[autonaut]

The 10 bolts refer to ring gear bolts, which you can only see by removing the cover coincidently using 10 bolts as well

 

[mebe007]

Refering to rustrockets comet it most likely is a 7 5 and you can use the later model (out of f bodys, s10's and a astro vans) 7.625" posi carriers but will need 28 spline axles also if the car had traction control you would need to remove the reluctor ring to fit in the 7.5 housing.

 

[DmcL]

I don't want to do any more than I have to for the sake of fitting a posi. So I need to look for a 7.5 inch posi diff for a straight drop in?

I'm knowledgeable about cars and worked on all my own but older American stuff seems very different to what I'm used to dealing with (older and newer German, euro domestics and j*).

I was looking at a nice complete 8.8 conversion rear end with posi and discs but it's like $1800 plus shipping which would likely be astronomical to Ireland plus I'd get hit with import duty of like 20%.

 

[darbysan]

Ok first I want to confirm what came stock in the 85 Monte Carlo SS. Was it a 3.73? Secondly my tach doesn't read correctly so what sort of rpm would I be spinning in 4th (200-4R) at 60mph and at 80mph?

The MonteSS came stock with 3.42 gears with a 3 speed Th350c and 3.73 gears with the 200-4r. Revs will depend some on your tire size, but with mine ( 200-4r and 3.73) I'm running 2500 rpm's at 75, and about 2000 at 60. The easiest swap would be a complete rear end out of another MonteSS with posi, but those may be in short supply where you live. Changing out the carrier for a posi carrier is not an easy task, unless you have some good tools ( dial gauge for one). It's easy to get the gears set too tight, or too loose, and then you have whine in the rear. It's one job that might be best left to a pro who has the tools.