It's more like 11 US gallons. Even with the longer sender the "reserve" left after the gauge hits zero is substantial. The longer sender is a vast improvement on the original type transforming the gauge from pointless to useful rather than all the way to anything resembling perfection. Also when filling the tank from a reasonably fast pump it is quite possible to hit the pump auto cut off with lots of capacity left in the tanks. Presumably down to filling faster than the balance pipe between the two tanks can transfer the fuel to the left tank.
