[siragan]

It's normal to go over the full mark.
You can pull out the gauge and re-zero the needle.
And fuel sending units are inherently inaccurate anyways. By poor design.

 

[siragan]

The way it works is actually quite ghetto. A little buoyant "bob" sits at the top of the tank and it drops from the top, when theta=90, to the bottom, when theta=0. Because of the width of the buoy, the resistor sends out a full signal anywhere when the tank is full to when the fuel level is equal to full minus the width of the buoy.

But instead of having me describe it, I suggest you go here instead.

http://auto.howstuffworks.com/fuel-gauge.htm