might post up some of the figures ive recorded over the past fortnight:
using Bp Ultimate (98ron) 380km from 58L (15.2L/100km) all "city" driving
using Bp Ultimate again 455km from 57L (12.5L/100km) this was 50% city and 50% highway and included our cruise on this tank, ive also noticed if i the odd boot or three i give it each day, over the course of the tank, ill loose anywhere upto 100km off the final kilometerage (?) of the tank.
one thing ive heard though, this is when i went to book my dyno a while back, it that higher ron fuel detriorates quickly, the fella told me to fill up the morning of the dyno run, as 98ron fuel will drop to 95ron levels in 48hrs and so on, the only way to combat this is to drop some octane booster in when you fill up, and ive done this a few times now, it makes a marked difference to the final L/km.