I have a Ford Fiesta Style 2006 which I bought (second hand) in May 2012. At that time when I filled up the estimated mileage available display on the dashboard was 409 miles. That estimate is now down to 362. The fuel usage (at the time of each fill) has gone down from 42.29MPG to 41.59MPG. This may be small but may be significant. I am not aware of my driving style changing over this period. A service (including possibly changing spark plugs) is not due until May 2013 How is the mileage available display calculated? Is there a link between that and the average MPG?