In the US, the 3.91 ratio was only used on the 4.6 and 4.8is and is harder to find. 3.0s all had 4.10 ratio. If still legible, the ratio will be printed on the sticker on top. Mechanically, they all boot in the same.

It may be easier to upgrade to 4.10s front and rear from a manual 3.0.

__________________
2012 xDrive35d
2001 BMW X5 3.0i 5MT
and two 1987 Corvettes - Callaway Twin Turbo and Guldstrand Grand Sport 80
Block Tester