IDing 9.25" Diff ratio and whether it's limited slip?

[Thunderdan]

Hello Late model B-body folks,

I'm trying to ID the ratio for the 9 1/4" diff on my '78 Monaco as I can't seem to fund any fender tag codes that correspond to the axle ratio. They came with either 3.21 or 2.71 and may have had limited Slip. Any way to tell w/o jacking the rear axle and spinning the driveshaft to count the tire rotation?

Thanks!

 

[pnora]

Who knows what is there after 45 years. Jack it up and tell us whats there.

 

[33 IMP]

Rpm versus mph should give you some idea. Wallace racing calculator should answer your question. As for sure grip, nope, gonna have to jack it up. You do know how to tell, right?
I would bet on 2.71s.

 

[Don Frelier]

I'll bet on 3:21

 

[Thunderdan]

Rpm versus mph should give you some idea. Wallace racing calculator should answer your question. As for sure grip, nope, gonna have to jack it up. You do know how to tell, right?
I would bet on 2.71s.

I would think that too, being that most pf the patrol models came that way. It's not yet drivable, nor does it have a tach (and I doubt a lock-up converter), but that's a tried-and-true method to determine.

Was hoping it would be a tag off the diff cover (ala GM often did), but I'll jack it up one of these days to check and see.

Thanks, All.

 

[33 IMP]

There MIGHT be a tag on one of the rear cover bolts, giving ratio and whether suregrip or not. Or, if it did have one, it might be long gone.

 

[Detective D]

From what I have seen, if you have limited slip it is probably a 3:21. Jack the rear and spin a wheel to check for limited slip.
And I think it is actually 2.76 for 78 in the 9.25". Maybe there are limited slip 2.76 but I have never seen one on a car. They did offer some for the 8.25.
Part of the police package was limited slip 3:21, I don't think they offered any other ratio with the 140mph speedo.

Being a 9.25", on the plus side you can make it into basically anything for not that much money. Having the proper housing for a late B body to fit the car right is the tricky bit

 

[YY1]

...except the later model (1990's and up) 9 1/4 has different splines.