I have a 2003 A4 3.0 Cabriolet. Since I bought it two years ago, it always scored 25-29 mpg, and when I drove very conservatively, 30 mpg. Suddenly the mileage dropped to 19-20, and it's remained there, steady. All measurements using the on-board computer, which I double-checked the old-fashioned way with the last tank. It's accurate. No "check engine" lights or dash light of any kind. No changes in fuel. The measurements are stable summer/fall/winter/spring. The dealer is stumped. Any ideas????? Thanks!

 

The biggest factors in gas mileage are the primary O2 sensors, mass airflow sensor, and to a lesser extent tire pressure. O2 sensors can be crapping out on you without being bad enough to trip a CEL. First thing I would do though is a three part seafoam treatment and check tire pressures (those are cheapest). If that doesn't do it, change the primary O2 sensors.