not sure, but I would guess its a combo of several things- engine breaking in, transmission learning, and the drive-by-wire throttle. Tell your wife to be a bit heavier on the pedal. I find that you need to be a bit more aggressive with your foot, eventually it will become second nature.

The hesitation seemed to lessen as I drove it more...now I rarely notice it.

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2013 X5 35i M Sport