There is nothing wrong with your car. I am a former GM engine engineer and I can tell you that is completely normal operation. Most GM cars clip the instanteous fuel consumption reading at 39.9litres per 100km in the Instrument Cluster (eventhough the fuel consumption may be reading higher in the Engine Control Module) and it is not unusual to hit this fuel consumption rate at higher loads when accelerating (especially at higher rpm where extra fuel is added, i.e. running rich mixture, to protect components like turbos and catalysts). On deceleration, the fuel consumption goes to zero as the fuel injectors are disabled (named DFCO - decel fuel cut off) this is to save fuel and avoid poor combustion/poor emissions at light engine loads, and as you approach low rpm/vehicle speed the injectors are reenabled and when you are travelling at such a low speed your actual litres per 100km will be really high: i.e. fuel consumption is 2 litres per hour consumption at idle, you are travelling at 5 km/h speed meaning that it will take you 20 hours to travel 100km, which equates to 2 litres/hr x 20 hours = 39.9litres/100km. Does that make sense?
Enjoy your LGX V6 they sound great!
