Making changes based only on a perceived difference in fuel efficiency over a short time like that is usually not a wise thing to do.I'm concerned though as mixed driving today on a fresh tank of gas yielded only 40mpg. I may replace the O2 sensor next.
Give it a little more time.
A 3 MPG difference can be caused by external conditions and measurement "error". Hot weather and running the AC more can account for 1-2.