Chevy & GMC Vans

So, I was given this front diff from a late model Safari body that was headed to the crusher. No doors or rear end so nothing to go on as far as actual year or any RPO codes.
What is the easiest way to determine the ratio, without tearing it apart. I think it is fair to assume it is either a 3.48 or a 3.73 ( or what ever the close numbers make it).
I have seen variations on locking one axle, spinning the input, and counting the rotations of the other axle.
If I understand it correctly, the other axle should rotate twice as fast with the one axle locked; so to find the ratio, I divide the free axle turns count in half , and then compare to the input revolutions, for the ratio.
Have I got that correct?
Thanks,
Rod J

Rod, take the cover off and read the numbers on the gear. If your going to use it you'll want to at least change the fluid and eyeball the gear condition anyway.

Woodbutcher,
As stated, this is a front diff; so there is no normal 'cover'; just the side of the aluminum housing where the passenger side axle bearings are located.
Rod J

WE7X wrote:If I understand it correctly, the other axle should rotate twice as fast with the one axle locked; so to find the ratio, I divide the free axle turns count in half , and then compare to the input revolutions, for the ratio.
Have I got that correct?

Yes, you have this essentially correct.
This method works perfectly, no need to take anything apart.
You can also turn both axles 1 turn, and count the input.
Same thing, less math. lol
Either will work.