I used to have a Nissan 350z, and those things commonly developed inaccurate gas gauges (mine included). Usually the problem boiled down to dirty contacts on the fuel level sender. To fix it, you could just take the sender out, wipe off the contacts with a Q-tip, and stick it back in. I did that and it fixed the problem for me. Seems like it's worth a shot since it doesn't cost anything.

__________________
2003 X5 3.0i, 150k miles