I reset the average mpg on filling the tank. Having travelled 405.1 miles I then refilled and calculated the fuel economy. The reading on the dash indicated an average of 54.1 however, on calculating my fuel usage (4.546 litres gallon equivalent) I was astonished at the accuracy as my calculations revealed 54.61 mpg.
Although using the same filling station the only variable could have been a different pump dispensing a slightly different amount - I always stop at the first click. Never had any car where the car's estimation was so close to real world results.