Why is my car's fuel reading so incorrect?

2016 Ford Focus, fuel efficient readings after a couple weeks go all the way to 99.9 MPG no matter how I drive, and on the estimated fuel left, it's used up at a rate of almost 5 miles to 1 actual mile, Yesterday I left the house to go grocery shopping and it had "100 Miles to Empty" and I drove a 5.4 mile trip twice, and when I got back, "60.6 miles to empty". It's the SE Sport model. Any help is greatly appreciated

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