An abrupt drop in fuel mileage usually signifies a bad O2 sensor. May not be bad enough to trigger the O2 MIL, but mileage can drop as low a 10MPG.
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72
'02 CLK320 Cabriolet - wifey's mid-life crisis
2012 VW Jetta Sportwagon TDI...at least its a diesel
Non illegitemae carborundum.