my fuel meter is always not accurate on the reading,

for example, when i fill a full tank, the meter will show "full" on the meter, then after i travel for a quite a bit, the fuel gate will still say "full" and then all of a sudden drops to below half tank.
looks like something is stuck.

just wanna ask whats the procedure for troubleshoot this, coz i dont want to go through a lot of fuel to find out what is wrong, i was wondering if it is the fuel sensor? or the fuel meter having the problem.

now i simply cannot trust what the fuel meter says and it is pretty annoying, wanting to get that fix. anyone have similar problem and know how to fix?

many thx.