A bad fuel tank sending unit is the most common cause of a fuel gauge not reading correctly. Over time, the electrical contacts corrode, the float deteriorates, and sometimes the unit itself can even start leaking fuel. The only way to correct these issues is to replace the sending unit. This sender is a direct replacement for the original unit. You will need to replace the sending unit o-ring at the same time to ensure a proper seal, and, you should inspect the lock ring for damage as well.
As shown in the picture above, this sending unit installs with the post with the black grommet up.