on a mini I owned previously my fuel gauge was correct from 1/2 full to full
but at the half way point it went straight to 0.  The unit in the fuel tank
(called a sender unit) has a bar that brushes allong a coil of wire to work
as a variable resistor, some of the coils had torn from all the brushing,
thus it worked to a point.. also it can have spots of wear which means at
certain fuel levels it will go down to '0'.
attach a multimeter set to 'resistance' accross the fuel tank sender unit
instead of its normal wires, then syphon from a full tank and see if the
resistance changes evenly.. failing this empty the tank and take out the
sender unit to inspect!

James