Gas gauge not working properly

I have the car listed above base Sedan. When my car gets low on gas, especially when it gets below a fourth of a tank, the gas gauge will say it had more gas in it than it does. And when it gets really low, it says it's full. I don't let it get very low anymore but this still happens. And every so often when I check to see how much fuel I've used, say it says I used 2.0 gallons of fuel. When I shut the car off and come about to drive it later and check to see how much gas I used right when I start it up, it'll say 3.2 gallons used without driving it. And then my Miles per gallon reading will go down too from 22.0 or smt to 20.0 because it says I used more gas when I really haven't. Is it just a bad sending unit or maybe something will fuel pressure. I don't know it's just annoying.