I wouldn't even bother to check the fuel consumption for the first 5 odd tanks. It will always be a bit worrying as the engine needs to free up.

Thing is fuel economy is such a personal thing.

It depends entirely on the roads that you drive on, how much traffic there is, how you drive them and how much of a heavy braker you are.

Just to make you all sick, I am averaging 46.5mpg in my Mondeo on the current tank. The tank has so far done 490miles and still has an alleged radius of 50miles. This is after a lot of town driving, and over 4 hours on the motorway with the cruise control set at 90.