The estimated range of miles on my car go up and down in seconds?

I have a 2009 Altima 2.5 and one minute it says I have 30 miles left to drive and another second it says 14 miles left then it goes to 21 all in seconds. Can someone explain to me what's wrong with my car? I'm sure this car isn't supposed to waste that much gas, unfortunately it feels like I'm Illinois up the tank for a Ford Expidition instead. Please help?

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