I would not lie or mislead you but I did nothing... I also wonder why the previous owner it was ~11.6km/L and when it came to me I did a detailed examination, changed every single lube... a couple times got HiOctane mixed..
One thing which I did right after recieving was 
1 - Resolved O2 Sensor Issue (it had error) it took lot of research to resolve it
2 - Complete throtle body was refreshed and serviced
3 - installed new battery
the car was reseted due to battery removal and after that the average cllimbed to 16km/L then slowly it made it to 19KM/L on meter while on practical calculations I have mapped it does 16~17km/L in reality even though meter shows 19km/L
one of my old pakwheeler friend @dr.omair is using passo too and he is getting 19km/L while showing 21km/L on meter his passo is relatively less on total miles and the fact he is soft on throtle he gets more mileage then me.