Mine averages about 13L/100km general driving around town, never actually done a nice hwy cruise to measure =S
I do keep a graph / spreadsheet to monitor economy.
Usually fill up about 35L at a time, never even get close to empty, leave any crap in the tank at the bottom rather than suck it into the engine...(so i was told many years ago, can't hurt)
I always use BP 98, tried a tank of Shell 98 and consumption went up to 14.6L/100km, so back to BP i go. Haven't done the math to see if it worked out cheaper per km but at a guess it won't be much different.