Double Dark Side! Diesel & 8P
What setting do you have your DIS on? It sounds like you have it set to monitor your current driving, like real time MPG which is why its up and down like a tarts knickers! You should be able to set the DIS to display MPG since the last time it was reset. What I tend to do is reset my DIS every time I fill up so that it calculates the MPG for that tank. Also the DIS calculates the MPG using the petrol that is still left in the tank! This means that when the petrol light comes on with 35 miles left in the tank the MPG calculation is using these 35 remaining miles but if you use a spreadsheet (like I do too) then your calculations wont be the same.
My DIS tends to show around 26mpg but when I put the milage or when I actually fill up into my spreadsheet I get around 22mpg! I tend not to give a crap any more.