Me too. Mileage dropped from 26 (my normal average) to 16, bam, in one week. And 18 the next week. Only reason I know is we put all our gas receipts in a spreadsheet. Tires are all 35-40psi and the 4 spark plugs I could get to were replaced last fall. I almost blamed my neighbor for stealing gas and then I remembered the gas door is locked
If you find an "open-at-8pm" place with the Denso or Delco 02 sensors let me know too. Sounds like a good place to start.
Doesn't sound unusual to me.
I average 15 mpg around town with short trips. Once warmed up, it will get much better mileage like around 18.
Of course it depends on how loud you like to listen to your engine roar. :P