It appears that the majority of 3.0si don't have gear problems (transmissions). Sorry to hear the gears and the gas gauge are off. Normally BMW doesn't have gas gauage problems (that used to be a very MBZ thing!). See if it gets better with your next fill up (which in an X5, will be Very soon!!
If not, then make an appt., and request/politley demand a BMW loaner. Good Luck.