Subaru have two standard sized diffs in resent years, one being know as the r160 and the larger Sti version r180. The r160 is a general fitment in all forester, Imprezas and Libertys, and has many different ratios available, also can be single or limited slip.
Here are some of the r160 ratios available:
3.54, 3.9, 4.11, 4.44
The R180s are not so common as the r160 rear diff on the Subaru’s , more commonly found on the later Sti models. It is sizably larger than the r160 and heavier, It requires the use of larger rear drive shafts and hubs.
Other ways of fitting the r180 is to use custom made shafts where the r180 inner C.V housings are used on the standard r160 shafts to make them fit in to the standard hub thus saving the replacement of the hubs and then braking system, This is not as simple as it sounds and it will need to be done by a drive shaft shop. This has proven not as strong as the complete shaft being used.
The ratios found in the R180s are 4.44 in version 4,5,6 Sti RAs and 3.9, 3.54 in the later GDB chassis STi’s (version 7 on).
The rear hubs on the R180 set up incorporate larger hand brake diameter and larger splined section for housing the larger shaft, available with the single, 2 pot Subaru caliper and Brembo backing plate and hole centre’s. The single and 2 pot Subaru caliper R180 hubs are less common as they are only found on most of the RA STi’s such as the V4 ,v5 and V6 sti RAs however note that they do not carry the abs hole for the sensor and the phonic ring for the abs signal so this would be inferior to a conversion onto a street car that has ABS.
Late model r180s such as the 06 Sti 3.54 have a larger pinion flange which will not suit a standard liberty, forester or Impreza tail shaft. The pinion flange can be replaced with one that is compatible with your tail shaft but be ware that the nut which holds it to the pinion is also the preload for the pinion bearings! Must be adjusted correctly.