theres lots of things that cause crap fuel economy
1) dead 02 sensor
the std 02 sensor will last around 50,000kms
after that it no longer provides useful feedback to the ecu
the ecu uses the 02 sensor to detect fuel emissions (or air:fuel ratios) and makes changes on the fly (to save fuel).
so if you have never changed it and the car has over 50,000k;s it might be worth checking - any mechanic can do it
2) state of tune
the state of the tune will play a big part. if its the stock ecu then there are defintetly gains to be had.
you can look at a remap, a piggyback system or a standalone and you will get better economy (power is a bonus too)
3) how you drive it
another killer for fuel economy is how you drive it.if you constantly load the engine up to bring it on boost its going to chew fuel.most cars are setup to be lean on light cruise but as soon as you load it up it will be rich as all hell
varying the throttle constantly will also prevent 02 feedback from work. 02 feedback needs a steady TPS signal in light cruise before it will do anything so if you keep moving your foot or accel->deaccel->accel->deaccel 02 feedback wont work. the best way to prove your driving is at fault is give the car to your girlfriend or mum for a week, i bet they can do 500km;s to a tank
4) what is a full tank?
some people call a full tank 45 litres,some say 50, same say 55 etc
as long as you keep using the same reference point its fine. the r33 tank is approx 55litres.
the most i have ever put in mine is 52 litres.
5) variance according to how empty u go
there is a big variance in how empty each person goes. some say near E is empty, some say at 1/4 its empty
some wait for the light to come on etc so each person has their own view of it . ive gone past E myself and it keeps going. there would be at least 30-40kms difference between just before E and going past it till the light comes on.
this enough to make a difference between the car doing say 340kms upto 380kms etc (my experience at least).
6) its a performance car
its not aimed at being fuel efficient etc but if you follow those simple steps it can be pretty good
my personal record was 542kms and it wasnt empty yet-from melb to sydney
7) fuel types
ive never seen any specific fuel type make a big difference in economy
some people swear by brand names etc but i would excpect around %1-2 difference in economy.
i dont see 100ron giving %30 more economy etc - same with other fuels
8) tyres
your tyres will certainly skew your results.
check your tyre pressure. 40psi on all 4 will give you the best comprimise between economy, traction, and control
lower pressure is great for grip and cushy rides but will have less economy