Sorry but that figure is impossible from a KV6. Fill the tank up at one pump, stop when the automatic filler shuts the pump off then when the tank is almost empty return to the same pump and follow the same proceedure. You can then calculate the true consumption rate.
Minimum consumption occurs with steady motoring, in top gear and at a point called the minimum specific fuel consumption. For a typical petrol engine this occurs between and 1/3 and a 1/2 of maximum engine speed. A power curve looks like the profile of a mountain, whereas specific fuel consumption looks like the valley between two mountains. The point at the bottom is the minimum specific fuel consumption and that is the point whereby minimum fuel consumption occurs when compared to power produced. This is a dyno figure, on the road many other factors combine to increase fuel consumption, air resistance, how slipperey the car is through the air for a given speed, rolling resistance (tyres, hub bearings), the weight of the car, drag that tends to pull on the rear of the car as it is pushed through the air, air pressure. Taking all these things into account I have found the best ecconomy from my 2.5 is achieved at around 2,500 revs in top gear.
This illustrates what a specific fuel consumtion curve looks like
