Have you got any aftermarket stuff on your car??
If so, the stock ecu might be having a bit of a hissyfit and could rich & retarding the tune/engine. In which case, you'll be constantly running rich, even when driving midly and hence you'll be using lots of fuel. Alternatively, your O2 sensor might have gone bad and might be malfuctioning. Considering Skylines now are 10+ years old, id could be a factor worth considering. As mentioned above, best taking first for a service with a diagnostics and tune. That way, you've got all bases covered.