I changed your title to be helpful. Please state what year and trim Civic this is and explain what it is you got. Details will help and so will research.
2010 Honda Civic LX fuel gauge goes from being full to empty and then flips back and forth between top and bottom. Not exactly sure what's going on. The only way I know I'm close to empty is the orange light that comes on whenever you get close to being empty
Im not familiar with digital fuel gauges but what I can suggest is to track your mileage per fill-up and go by that until another member chimes in or you find the answer after research.