I've got a R34 sedan, stock standard. 35 PSI tyres.
Do quite alot of highway driving and get very close to 600 km's a tank quite frequently. NO thrashing.
If I have a heavy foot that week, I get approx 450-500 km's.
I use to get quite low milage as well. About 400-450 km's. I just gave the car a oil change, air filter and spark plugs and could see a difference immediately.