False range

Had my Clio 197 for almost a year now and it's always seemed to have a problem knowing how much petrol it had left... once it gets down under a quarter of a tank it's a lottery as to when it decides to run out (it's usually died between 80 odd miles and 50). I've been ignoring the problem by keeping a full Jerrycan in the boot all the time but does anyone have any ideas as to what could be wrong?