Diameter of the drive shaft has nothing to do with differential ratio. It's the relative diameter of the pinion gear verses the diameter of the ring gear that determines the ratio. My '06 has the factory stamped differential cover, and on it is a white sticker that states SYNTH LUBE ONLY, a bar code and below all that is 3.55. The 3.55 is my gear ratio. If your tag is missing or you still want to check, then... Support the car on jack stands with rear tires in the air. Mark the drive shaft with a chalk line or piece of tape in-line with the driveshaft. Mark one or both your rear wheels with a chalk mark or tape. Have your lovely assistant watch the tires and you slide under the car. Rotate the drive shaft by hand. (Car in neutral, parking brake off) Count the rotations of the drive shaft carefully and do your best guess for partial rotation. If you don't have a lovely assistant, just mark the tires on the inside where you can see them. When one complete revoulution of the tires has occurred, STOP. Here are some common ratios
2 3/4 turns = 2.73
3 1/3 turns = 3.33
3 1/2 turns = 3.55
3 3/4 turns = 3.73
almost 4 complete turns = 3.90
just over 4 complete turns = 4.11
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