HELP -- Car Mileage Question....(Infiniti Q45t)?

I think I might of found the problem. I have your same car, with the same problem. When I'm on E I've only driven about 200-220miles. Howeverwhen I fill up, I'm only putting in about 16 Gallons of gas, ah ha!! We have a 21.4 Gallon tank. Our problem most likely is our "fuel sending unit" or "fuel gauge cluster" im told. So I'm getting my car diagnosis for that later this week, but I mean it's like the only thing left to check. Hope this works!!!!!