hmmm this is what i get.
E85 18.6l per 100km. 335rwkw.
BP98 13.2l per 100km 275 rwkw.
the way i see it is, if you want to save money on fuel a skyline is not the best car for it.
but for the OP the only real way to find out whats going on is to put it on a wideband 02 sensor and see the the cruise AFRs are.