Differential Gear Ratios
All E10, E21, E30, and E36 4-cylinder models (except M3) fit with small-case differentials with a ring gear size of 168mm. Accordingly, ring and pinion gears can be exchanged freely between all these cars. Gear ratios commonly available in the U.S. are 3.36 (2002 Turbo), 3.45, 3.64, 3.91, 4.10, and 4.27 (very rare).
Most 2002s came with a 4-speed transmission and a 3.64 gear ratio differential. Non-California models from 1976 were fit with a 3.91 ratio differential. Nearly all 2002s did not have a limited slip differential.
E21 models fit with a 4-speed transmission had a 3.64 ratio differential. E21 models with a 5-speed overdrive transmission had a 3.91 ratio differential. Accordingly Aardvarc recommends installation of a 3.64 LSD if the 2002 is fit with a 4-speed transmission, or a 3.91 LSD if the 2002 has a 5-speed overdrive transmission.
For competition cars, Aardvarc recommends a 4.10 or 4.27 ratio LSD. For cruising (engine running at lower RPM at higher speeds), we recommend a 3.64 or 3.45 gear ratio LSD depending on the transmission (4 or 5-speed).