What Tink says

Wind in particular has an *enormous* influence, and is probably the culprit in day to day variations that are seen.

E.g., a 5 miles per hour tailwind driving 69 miles per hour will increase your mpg from 64.78 to 70.53 ! I think the change approximates car speed in the opposite direction, but really have not played around with the simulator enough to be sure. So if you drive into a 10 miles per hour wind at 50, your mpg will approximate driving 60 miles per hour without any wind.

New Mexico gets pretty windy, so I have taken to looking at the weather report to try and time my trips. I know -- that is rather extreme -- but it makes quite a difference.