You'd have to be caning it on a track to get those sorts of numbers. I suspect your trip computer. As suggested, recalculate independently of your fuel computer, using your odometer and recording fill-ups for a few fills. My total fuel consumption, mostly in crappy Perth traffic, is 12.2 l/100 km, over the past year. I'll easily do 9.2 l/100km on open road trip at speeds up to anything that local roads can sensibly stand in short passing bursts, and have seen it down to 8.5. 6th is a wonderful gear for economy!!
CP_e Standback & PNP; CP_e 3" SS Downpipe; Corksport FMIC with Top-mount K&N filter & OEM Ram CAI; Turbosmart BOV; Dashhawk; Prosport Boost Guage; JBR solid shift bushes; DBA 4000 Wiper-Slot front rotors; Hawk Ferro-Carbon HPS Street front brake pads (@ 69,000km); Sumitomo HTRZIII's in 225/45 x 18