My gas milage is usually 18-22 MPG (mix of city and highway), but I have noticed that if I let my car sit for long periods my milage decreases. I filled up today and my milage was 16.4 MPG. My last fillup was Feb 22. I had several trips out of the country so my car sat in the garage for three weeks at a time. I don't think this is long enough for the gas to deteriorate, so it might be evaporation, but I thought the tanks were not vented. Has anyone had a similar experience?