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?

Don't worry at all about your mpg going down, it always does in the winter. Just make sure that all tyres are properly pumped up, there's no extra weight etc, and if all is the same, its the weather. When you turn the car on and demist the windows etc, you're wasting fuel. It should start to increase again soon :).

Also, I presume you're working the figures out manually? Not using the trip computer I hope, as these are horribly inaccurate.

The MPG had dropped on the display in my A4 1.9 diesel, It was about 45 average but now 41.5, If you floor it then normally it will knock 10 or 20 miles off instantly off the distance left and then if you take it easy it goes back up after a minute. so it's constantly fluctuating depending on how you drive at that particular moment.

They are not very accurate most of the time . Lower MPG is expected with cold weather though