Most fuel consumption comes from acceleration not your actual top speed. If you do a lot of suburban driving with traffic lights and roundabouts that chews the fuel. Cars are designed to be driven at 80km/hr. My 07 Calais V does 11.5 over the long term. V6 should do between 11 and 12 on average. If you think your fuel sensor is wrong then run down your fuel to half way on the fuel gauge and reset your trip and fill up your tank taking note of how much fuel you have put in. Drive until it gets back to half way and see how many kms you have driven. Fuel from pump: eg 30L distance: eg 300km ... 30/250 x 100 = 12.5 L/ 100km That will be your actual average fuel consumed over 100km.