What is your water temp like? Is it staying on the cold side of 80 degrees and has your heater output diminished? If so, the problem could possibly be your thermostat. If the thermostat is either opening early or not closing properly the water temp will remain low which tricks the engine to stay enriched causing poorer gas mileage. I was getting 17 miles before Christmas. In January I noticed that my water heat was barely making 60 degrees and my mileage was about 12-13 miles/gal. Switched in a new thermostat and I'm back up to 17 miles again. Another avenue to explore.