fuel gauge not working

has anyone else had this problem or does someone know what the fault is.

my fuel gauge does not work and the reading on the maxidot display for range left before empty shows---- when i fill up the gauge goes to full and the reading on the display shows 2000 miles left which then starts to decrease to 1800,1750,1580,1200 etc.after about 10 miles the gauge then goes back to empty.