GAS mileage is messed up

[DJ11]

Every time I would fill up my tank, it would just say I have a half tank. It would change every time I would drive. Sometimes it would go back to being full and then when I turn off my car and turn it back on, it would say I have 190 miles left before fill up on a full tank of gas. Is something wrong with my computer? Or
my gas tank. Thanks