its interesting a thread like this has come up, i always get about 400Km to a tank, maybe 350km if i have been on boost a lot, this is city driving (i never country drive).
On saturday when i was at my mates trying to cut the lock wire (i still don't know what colour if anyone can help) we had a fiddle with my blow off valve. its a HKS SEQ blow off valve and we undid the screw on top another turn, now it seems to blow off much louder and more often? it sounds like it is bleeding boost while i am driving but when i look at my boost gauge it says i have vacuume. Anyway i filled it up that day and i still have about 1/3 maybe a bit more of a tank left and i have done 350Km. so i will prob get about 466km out of the tank.
i thought i would get less km out of the tank by doing this. my mate hasn't looked but he said if i have a map sensor it may save me petrol?