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look in the glovebox or on the inner fender well under the hood to find the build sheet. Will probbaly tell your gear ratio. Though it may just say stock ratio. The absolute best way to find out is the remove the diff cover and count the teeth. Never hurts to change the gear fluid anyway.
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RPO Ratio
GM1 2.59:1
GU2 2.73:1
HC2 2.76:1
HE3/G44 3.07:1
GU4 3.08:1
GU5 3.23:1
GU6 3.42:1
GT4 3.73:1
GT5/GT8 4.10:1
HC4 4.56:1
most likely 3.42, but could be 3.73 or 4.10
this, however wont show you if they've been changed before.