You are correct: It matters-NOT! What matters is not MPG (Miles Per Gallon)-or kilometers per liter, but instead MPH (Miles Per Hour) and MPD (Miles Per Day).
17,4 liters/100km. This is my average fuel consumption with 3200 kms. I did only 1000 kms break-in, and since then ,I have been driving the 997s very hard. I think fuel consumption is too high.( My average fuel consumption in my Boxster S was 14,2 liters/100 kms for 25,000 kms)
I'm about 27mpg on a long journey which I think is okay. Around town (which unfortunately is where I do most of my driving) I get more like 10-12mpg....