[Shotimes] gas gauge off by 1/4 tank

[van Oss]

My wife's Taurus (an LX wagon with 3.8L and 18 gallon tank) has a very
inaccurate gas gauge.  It is pessimistic (reads lower than actual fuel on
board) by about a quarter of a tank.

Is this most likely to be an issue with the sender in the tank, or in the
dash gauge itself?  Or perhaps in the electronic module behind the
instrument cluster that is responsible for anti-slosh and low-fuel lamp?

What if anything could we do to make the gauge a bit more accurate?  TIA

Joseph van Oss