first thing to do is to not start looking at how far you are getting "on a tank" and start looking at what your actual economy is. put it into litre per 100kms because that will give you a better idea of how poor your economy it is and whether your fuel gauge is just a bit dodgy. things to test with the car will be the O2 sensor (can be done with a multimeter or a consult cable) and take note of how long the car takes to warm up when driving. the car should come up to temp (middle of the temp gauge) within about 5 mins of driving. if it takes longer than that, or doesn't come up to the middle of the gauge, then the thermostat may be dodgy and this will cause excess fuel usage.
and peak hour traffic is going to kill your economy. driving style will have the biggest effect on economy. light on the throttle. coast as much as possible. if there are a few lights in a row and the next one is red, don't floor it to the speed limit and then get hard on the brakes. just cruise along at 30 or 40kmh, then just lift off and cruise to a stop.
having said all that, i'm guessing (without knowing how much fuel you are actually putting in) that your economy is probably only around 13L/100kms to 14L/100kms, which for an auto skyline in peak hour traffic isn't too bad. it will never have the same economy as something like a 4 cylinder hatch, as they are a smaller, detuned engine in a lighter car.