your not serious are you? baha a tank a week is pretty darn good, i get 500 km to one tank (55l in an r33 not 65)
and if your thrashing it around half a tank in a night is very easy.
i think drive your car for a while before you decide things are broken.. when i fill up my gauge will say on full till about 150 kms then suddenly drop to 3/4 full. same with my cousins and a few other people i have spoken to.