Sorry to revive an old thread but after spending hours researching rear Subaru LSD ratios. There is a lot of conflicting information primarily from people confusing the front and rear final drive ratios. This thread comes up on search a lot and has a ton of views. I wanted to share what I found:
Subaru WRX STI rear differentials:
2004 3.90 ratio mechanical LSD
2005 3.90 ratio mechanical LSD
2006 3.545 ratio mechanical LSD
2007 3.545 ratio Torsen LSD
2008 3.545 ratio Torsen LSD
2009 3.545 ratio Torsen LSD
2010 3.545 ratio Torsen LSD
2011 3.545 ratio Torsen LSD
2012 3.545 ratio Torsen LSD
2013 3.545 ratio Torsen LSD
2014 3.545 ratio Torsen LSD
2015 ????