Miles to empty on Lincoln town car is inaccurate

Since I bought it, I wasn’t having this issue
until a month or 2 later, but now every time I
fill up the tank, the miles till empty have been
decreasing when I fill the tank up.  For
example, when the tank was full, I had
340miles until empty, that’s how many miles I
had when I bought the car. Now the problem
is that ever since, that 340miles till empty has
been decreasing or has decreased even when
the tank is full. When I fill up tank now, it says
I have 130miles until empty, which didn’t
seem right. The miles till empty has slowing
been doing that when I fill up my tank. It had
gone from 340 miles, to 320 miles, to 290
miles, and so on and so forth. You have any
idea what that indicates? Or what can I do?