Look in the main folder for that airplane and check the fuel section of the aircraft.cfg file.
Here's the Fuel section for the default A-321.
[fuel]
Center1 = -4.0, 0.0, 0.0, 2150, 0 //Longitudinal (feet), Lateral (feet), Vertical (feet), Usable(gallons), Unusable (gallons)
LeftMain = -6.4, -16.8, 0.0, 2040, 0 //Longitudinal (feet), Lateral (feet), Vertical (feet), Usable(gallons), Unusable (gallons)
RightMain = -6.4, 16.8 0.0, 2040, 0 //Longitudinal (feet), Lateral (feet), Vertical (feet), Usable(gallons), Unusable (gallons)
LeftAux = -15.2, -37.6, 2.0, 780, 0 //Longitudinal (feet), Lateral (feet), Vertical (feet), Usable(gallons), Unusable (gallons)
RightAux = -15.2, 37.6, 2.0, 780, 0 //Longitudinal (feet), Lateral (feet), Vertical (feet), Usable(gallons), Unusable (gallons)
See what they have in the "Usable" field (4th field) for each tank in that 777. Add them up and see if the total looks reasonable. There must be a data error somewhere.