ive just calculated it by my last tank which i got 470k's when the petrol light came on (my car still needs a tune). i calculated 13.8L/100km, so thats pretty average for skylines.
if you're looking for excellent fuel economy you've bought the wrong car. 480 is good out of a skyline. dont forget its a turbo 6cyl. you should only compare it to other cars in its class.
i get approx. 350k's out of my lancer (40L tank), and 300ish k's out of my m3 (60L tank, but its running an un-tuned EMS computer).
if you want something that wont eat up all of your petrol money you should think about buying a weekday workhorse.