During my 8000+ mile rune to the rally and back I noticed my mpg dropped from about 50 to 30. I stopped at Sturgis BMW and they told me the code informed them that my O2 sensor was toast. When home, I did a major tear down and did a lot of maintenance things. I replaced the O2 sensor with one recommended on this board. It was for a 540 BMW auto ($100 vs the $367 oem one) and was the right length and had the correct plug. The issue now is, it runs great but the OBC says that I am averaging 38.3 mpg. This mpg is not pulling a trailer or running things hard at all. Does the code need to be cleared by the dealership or what is going on? At the tear down i also replaced air filter, spark plugs, fuel filter, throttle cables, installed HID lighting and general fluid change in additional to the O2 sensor.