fuel economy dropped

I have had my '06 325i for two years now, I have put about 35,000 miles on it and always average about 28 mpg. In the last 6 weeks mileage has dropped to about 25 mpg and staying steadily there.

No changes in my driving. No codes or warnings. Car runs as good as ever. Currently at about 117,000 miles. I am going to replace the air filter but struggling to think of anything else.



Any thoughts about the longevity of O2 sensors? Just thinking they can change performance before getting far enough out of range?



Thanks,



Kurt