My Ion does not give me an accurate gas reading. What could it be?

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I filled up my tank last night no more than a mile from my house, did not go anywhere after that. When i turned on my car this morning it was missing almost a 1/4 of the tank. So i went back to fill it to the top again, and it only allowed me to put in $0.40. I noticed this a while back, especially when it gets below a 1/4 tank, it seems as the gas evaporates form the reader. I ran 7 days with my "LOW FUEL" sensor on. I think the floater or gas pump might be worn out, or even broken. How can i figure this out?