Call me sad, but I do like to keep my fuel receipts every time I put petrol in the Ghibli. The idea is that I use the information to see if there's any problems happening with the car which might show as a reduction in fuel economy. However, this takes far too much discipline, so in reality I only update the stats about once every two years!
As you can see from the chart, on average I've been getting 7.6km per liter of fuel (the gradient of the line). Expressed in more familiar terms, this is approximately 13 liters/100km or 22 miles/gallon (UK gallons), or 18 miles/gallon (US gallons).
Seems that my fuel economy has been getting a little worse recently, so I'll continue to monitor it and update it more frequently, particularly since having the big service performed and running on higher octane fuel (which should run less rich).