If your car is stock and HAS NOT had any aftermarket ring & pinion replacement, you can determine what ratio is in the differential by looking on the RPO list on the door of your car.

98 Auburn, 99-02 Torsen
3800-V6 5M GU5 3.23 Standard
3800-V6 A4 GU4 3.08 Standard
3800-V6 A4 GU6 3.42 Optional
LS1 M6 GU6 3.42 Standard
LS1 A4 GU2 2.73 Standard
LS1 A4 GU5 3.23 Optional
LS1 A4 G92 3.15 (Vette optional)
28 Spline Axle
7.5" 10 bolt Gasket Fel-Pro # 55072