I've been dailying my R32 GT-R now, usually driving in traffic. Usually get 450-550 km per tank (60L) depending on how much boost it sees (not much) and more importantly how the AFMs feel lol. I got 650km once, but I did lots of highway kms and the AFMs were kind. Engine is standard, standard turbo, standard dumps and aftermarket everything after.
My goal is to get 10L/100km, which I believe is achievable with Nistune, new coilpacks and new AFMs.
You should be easily getting 550-600km to a tank if you take it easy. R31s are lighter and don't have 4WD.
When I had my R32 GTS-t, I was easily getting 600km per tank in traffic. Only mod was R34 turbo, everything else was standard including the exhaust. Damn I miss that car especially the sunroof lol.
Biggest factor is your right foot. If that's in check, then it'll be O2 sensors, spark, tune. Also if your car is an Aus-spec, make sure the rear calipers aren't dragging.