Did you recently have the engine serviced? Honestly, 30 mpg is not bad. I have an 02 325i with 78k miles and I get average of 25 mpg city/freeway.
I would imagine that an abrupt drop like you mention would have something to do with computer compensation, if you are not noticing any power losses, etc. Have the codes run at your dealer if it's still under warranty.
2002 BMW 325i