When I first bought my 2000 BMW 323i two years ago, I solved the main problem the previous owner couldn't fix by replacing crankshaft position sensor. I put in a Mayle (made-in-Germany, but non-genuine BMW) brand sensor I pulled off another car for $20, and the ignition misfires went away.
I lasted me until now. The sensor started acting up again, and my check engine light came on. Although there were no misfires, I did notice my gas milage slowly decreasing, sometimes averaging as little as 22 MPG even with mostly highway driving. People online reported that genuine BMW sensors tend to last longer, so I decided this time to spend the $120 on a new OEM sensor.
Right away I noticed my car running stronger. After my first commute to work, the fuel efficiency reading on my dash went up to 31.3 MPG! The highest I've ever got on this car was 26, and that was on long stretches of open freeway between LA and SF. Getting around 30 MPG in a 12-year-old BMW during a daily commute makes me happy.