My exhaust tail pipe is covered in black soot. I have read that this could be due to a car runing too rich? That may be the case with a fault o2 sensor I believe...but I get great gas mileage, around 35mpg regularly, and would not think that a car runing too rich would get 35mpg. Would there be something else wrong with my car to produce the black soot? I don't race and I don't ever see black smog coming out of the tail pipe.