I'm with Dojo. Right about now (late october/early november) is when they switch over to oxygenated gas, which means terrible mileage for all!
You should see a label on the pump with something like "Between October 31 and April 1 the gasoline dispensed from this pump is oxygenated" yadda yadda. If you also see a label that says "Contains up to 10% ethanol" be prepared to never see 40 MPG again!
(at least until summer)
I'm still going to try and keep up my 40.5 MPG average thogh.