This paper describes how to realize an accurate, compact, and economical fuel gauge by the pneumatic method for deformed and flexible tanks which shake and vibrate, whose attitude changes and in which liquid sloshes and the temperature changes.The basic concept employed in the pneumatic method involves the use of the ideal gas law. The volume of liquid in a closed tank can be obtained by subtracting the volume of gas in the tank from the tank capacity. To determine the gas volume, both a reference tank attached to the main tank and the main tank were subject to the sinusoidal displacement. From the equations based on the dynamics of the pressure in the main tank derived from the ideal gas law and flow of gas thought a pipe, relations were obtained that gave the gas volume in the main tank from the filter pressure signals in the both tanks. The pneumatic method can solve the essential problems incurred by the shape of tank, i.e., deformation of the tank, but no other practical problems incurred when applied to an automobile fuel tank were investigated.Here we propose a new gauge based on the pneumatic method and investigate performance of the gauge from the practical view points of accuracy, size and cost. Validity of the proposed gauge was examined by experiment.