Fuel Gauge Telling Lies!

So my fuel gauge is telling fibs, its the most annoying thing
in the world!
It will say a quarter of fuel left,then takes 20 litres an shes
full!
Car is a 73 dart.

Any ideas on how on how to fix this?
Sick of driving around with a jerry can in the car!
I live in aus,hopefully i dont have to get a part
from the states!