Outboard tank fuel gauge

There's a handle on one end of those tanks that's meant to be used as a complex sightless level indicator. You grab it and apply vertical force, the tank will then pivot of the filleted vertex at the opposing end, causing the fuel in the non-baffled void space to "slosh" which provides direct reaction forces to the handle. Your arm acts as a PID logic unit, adjusting the applied vertical force based on the feedback it gets from the handle. Once balance is achieved, the precise level of fuel in the tank is calculated based off of the brains inherient knowledge of the void spaces shape and level of fuel in the void space.