Maybe have a look at a wiring diagram, if I remember correctly someone else had a similar kind of problem and it was found out that the speedo signal actually comes through the ABS module and not directly from the wheel sensor. That still doesn't really prove that your wheel sensor signals are good though.

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1988 325is (purchased new) sold
2004 X5 3.0 2005 X3 2.5
2008 X5 3.0 (new to me)