The humidity has an affect on my MPG. I noticed that my MPG went from 49 - 52 MPG down to 44 - 46 MPG when the heat & humidity started here in June. Yesterday a cold front came through and the humidity dropped. I filled my tank up and noticed my MPG is back up to 49 MPG, so the only thing that is different is the humidity. WOW the cold & humidity does affect the MPG (approx 4 - 6 MPG).