I wasn't having a go at you personally mate. Sorry if it sounded that way. Its just that the trip computers do throw out some interesting numbers sometimes.
Likewise Paceman, I wasn't having a personal dig, but the trip meters in many cars are not particularly accurate.
The only way to accurately calculate fuel economy is to fill up, drive til tank is near empty and refill. Work out how many Kim's are travelled & how much fuel was used.
The more distance you travel, the more accurate the calculation.