results on Ultimate 98, for a bolt-on equipped car
mainly city, light throttle = 300-350km on a tank
" " hard accel more frequently = ~250km
mainly highway, no boosting = 550+km (i reckon i could get well over 600 on a long drive at continuous speed)
hooning = very few km/tank
they'll never match a Yaris for economy, but like Steve said, it's hardly the point of a GT-R.