[RCSpeed]

First, look on gear shift selector and see how many gears you have. Either you have 1-2-D or 1-2-D-OD. The first is the 4T40 and the second is the 4T60. The 4T60 is in 2 versions. A straight 4T60 or an electronic shift version which is 4T60E. Also, look on the bottom pan and see if the tranny number is there. In addition, the 4T40 has a smaller side pan (drivers side, covers half the tranny) and the 4T60 side pan covers the whole drivers side of tranny.
Your car probably has the 4T60 because of the 3.1 but you never know.