Might want to address any traction issues from higher gear ratio with choice of tires. Or, see what happens before going to lower ratio. That is, unless top speed is your goal rather than A to B time. With lower gear ratio it will take longer for turbo to spool up and torque is your best friend when trying to get a heavy vehicle going. You should be able to put 600 ft/lbs to the ground with 3.91 ratio. Worst case you just adjust throttle input until you get rolling a few feet.

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X5 4.6 2002 Black Sap, Black interior. 2013 X5M Melbourne Red, Bamboo interior
Dallas