Fuel tank sending units are more or less are like potentiometers. You can do little math and design a program on microcontroller and display on seven segment. This would not be as precise...... If you want it to be precise you need to calculate area, depth of your fuel tank and degree of freedom your fuel sending unit can move in one dimension. Then you would have to need liquid level indicator.
these readings would fluctuate when your car is moving because fuel would not be steady and also take into account the evaporation.
above is the answer to your question........ i believe its not worth it the effort.
