The Petrol guage is a little random. However when I am running low my digital guage on the RPM counter displays a warning and flashes whilst counting down the miles to go before empty, this is very accurate. I also have the Fuel Range turned on within the MFD. My average is around 170 to 200 miles a tank during normal usage. I can stretch this to 240 on long motorway runs.
Hope that helps.