What kind of numbers are you seeing? My average MPG readout is actually the most accurate I've had in any car - usually within a couple tenths. The instantaneous readout seems normal as well, ranging from the single digits to 99 MPG.
Mine is maybe 1 mpg optimistic but I use it as a consistent reference point. If you are experiencing readings that are really way out of whack, then consider having your dealer service department investigate it.