I'd use external strain gages or a load cell on the bottom support for gas measurement.

The jarring around on rough roads or the changes caused by
acceleration and deceleration, or for an airplane, in rough air or
during maneuvers, would result in wildly varying readings that would
need averaging circuitry. Further, the tank mounting would have to be
set up so that the tank's weight is all on the strain gauges, without
interference from clamp or strap tension, and the tank would be
difficult to mount securely. I suppose a double bottom with the strain
gauge between the two layers might work.
The capacitance-type gauges use the dieletric constants of air
and fuel. They're different, and the device's capacitance changes as
the fuel level changes. Several of them spread across the tank are
connected together and the capacitances summed to get a much more
accurate picture of the fuel quantity than a single-point sensor can
give. No moving parts, very light and reliable.


Dan