Simrad GO XSE Operator's Manual page 29

you manage to achieve an efficiency better than your nominal
efficiency, you will be somewhere in the upper green zone.
Nominal fuel economy can be entered in the Vessel Setup dialog
displayed from the Fuel settings dialog.
You can reset your average fuel economy from the Reset Fuel
Economy button on the Fuel settings dialog. When you reset it, the
system starts calculating the new average.
Set the measurement units for the fuel economy gauge in the
Economy field in the Units settings dialog.
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Customizing your system | GO XSE Operator Manual