It is a common fault unfortunately.
My fuel gauge doesn't read quite right but once you know that you can just keep an eye on the distance covered and guesstimate.
Best place to start on fixing this issue is a simple clean, ....but obviously you will need to get at the senders to do this which isn't so simple unfortunately. But doable.
Link: http://www.350z-uk.com/topic/54763-how-to-fix-the-inaccurate-fuel-gauge-pdf/
I'm just living with mine for now.