Fuel miles

[Leon Fairbank]

have a 2006 Audi A3 3.2 Quattro, recently the fuel mileage on the dash has been defiantly showing the incorrect miles, I have a 55litre fuel tank and today I had filled up a full tank which should give me 315 miles in the tank, but instead the menu is telling me I only have 235? The fuel gauge is showing full but the menu miles seems well off, anyone know the issue or even if there is one?
Thanks

 

[vag-tech]

This is calculated via instantaneous and short term mpg.
What does it read when cruising?