Ignore the figures on the dash showing averages. I've been getting about 11.5 l per 100 km. Thats a combination of city/highway and about 1 tank per week - generally only 65 liters going in on a refil (suspect gauge doesn't quite read right). I use an iPhone app to record when I fill up and the distance, price etc.
The only way to establish what the car is really doing with economy is to track the fuel usage over a period of time. Generally your driving will be the same week to week in terms of distance, traffic encountered, route etc. So the best thing to do is monitor how often you fill. Ridiculous figures for economy indicate an issue - good pickup with the sensor replacement.
I've seen new VZ's getting average of 12 others with average of 16. Similar drivng techniques and distances etc. Holden were of the belief it was within tolerance and therefore not an issue. The cars didn't change over the 4 years we ran them - some were good, some were bad.
I've found the Caprice runs much smoother and more economically than the poverty pack I drove for the previous 4.5 years. I put this down to increased power and a 5 speed transmission.