Look, I drive for an average of maybe 50 miles on a new tank before those dash lights even start gong out. Sometimes the last one lights up when I fill up, sometimes it doesn't. And by the time those lights register that I have half a tank left, I'm always well beyond halfway to the 320 or so miles I usually get before my fuel warning starts kicking in; it's almost comical how quickly that last indicated "half tank" goes away. Those LEDs aren't even close to a linear representation of how much fuel you actually have and basing MPG off of them is a fool's game. 
The bottom line is that there are only 2 times that you can be sure of exactly how much fuel is in your Z; when the pump stops you from overfilling and when you stall out from going too far between fill-ups. Everything in between is just a bad guess that I wouldn't put any confidence in whatsoever.
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2009 370z #206 (all mods are Home Depot)
2009 Ducati 848 (full Ohlins, Termignoni, etc)
2003 Grand Cherokee (long arms, big tires, 4.56, lockers)
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