The speedo gets it signal from one of the rear wheel speed sensors. It will be correct after changing the diff. I've had old cars where the speedo reads off the transmission, but that is not what the X5 does.

The fuel mileage will probably be affected, but I suspect your driving style will matter more than the diff.

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David.X5
2001 X5 4.4i Sport
SOLD! at 160k miles