Sounds more like the wheel sensor input is not working correctly.

Go into the 'hidden' service screen for the nav (at work ATM so cannpt recall the key sequence) and whilst driving check the wheel sensor field increases/decreases with changes in speed. If it reads 0 all the time then the wheel sensor input is either U/S or you have a mis-wired loom (some were like that from the factory).

Nav inaccuracies of up to 100 meters nearly always indicate a Gyro or Wheel Sensor issue. If you were getting gross errors and no GPS displayed in the top RH corner of the map then I would suspect antenna.

See http://www.xoutpost.com/articles/all-...formation.html for more info on the problem, and the remedy. The article underplays the improvement in accuracy somewhat. There was a marked improvement when mine was fixed. And it isn't just X5's, mine was an E46. Go check the readings and the wiring,particularly if yours was a retrofit but factory fit nav's were also affected (but not in such great numbers).