agree with stephen...damn good liao.....
sorry, i know its not helping you by saying that, and i honestly dunno whats causing your higher petrol consumption. I got a rather heavy sub myself and twice so far, i can clock 13.5km/l usually its about 10+km/l
BTW, any one of you guys here getting low fc even when being light footed? <2k rpm? Cos i kinda notice my fc actually gets better if i rev it.