you should be able to determine the gear ratio of your current rearend one of three ways:
1. door tag
2. rear end tag (should say something like 3.L27 if its currently 3:27 gears, if its 3.L55 (3:55's). 3.L73 (3:73's) and so on and so forth. the L indicates locking differential
3. jack up the rear of your car and rotate the tire/wheel one full round and see how many times your drive shaft rotates...you can make a "white out" mark on both to determine how many revolutions.
with an automatic most prefer 3:73's on a daily driver other more aggressive drivers prefer 4:10's.