Unless someone stuck a long ranger in there while you weren't looking I'd be taking that back to Nissan.
Although the fuel gauge is a simple piece of technology and fairly easy to fix if it's anything like other manufacturers you may not get much pleasure from hitting Nissan for a fix but it's worth a try. Over the years I've had problems with fuel gauges in Holden's, Ford's and Toyota's which the manufacturers have never fixed properly, never had the issue with a Nissan but they should be able to fix it under warranty, it's whether they will.