mine always reads 1/4 full when off even if there is a full tank or less than 1/4 in the tank.
as for the gauge. mine was messed up when i first got my car. guy said not to fill it up cause the gauge will read messed up.
well i didnt fill it up but i put $20 in (like 7 gallons) and it read full. i go hmm ok maybe the guy was right. a few days passed and i noticed the gauge was reading correctly.
the sending unit is infamous for breaking. mines been on its way out ever since i got the car. best thing to do is just measure how much gas in it and use the trip odometer.
if u dont know how much gas in it either just run it till it dies or break the lazy chain and drop the tank, replace the unit