fuel economy fact or fiction

im interested in peoples opinions on how fuel economy figures should be achieved.
im curently doing figures on our car at the moment and they all seem pretty close.

Fuel figures on lacetti.
Tank 1: 610 kilometres 51 litres.300 hwy
8.1 litres per 100
11.9 kilometres per litre

Tank 2: 440 kilometres 39 litres. all city
8.8 litres per 100
11.2 kilometres per litre

Tank 3:600 kilometres 52 litres all city
8.6 litres per 100
11.5 kilometres per litre

Tank 4:570 kilometres 50 litres all city
8.7 litres per 100
11.4 kilometres per litre

now theese figures have been taken over 4 tanks some where taken over the school holidays when the mrs didnt need the car and i traveled to work home for lunch back to work then home again over 2 weeks with some running around inbetween.
but most was with the wife driving school/work/me ect doing lots of driving.
around 500 per week all local.in bendigo every thing is close.
people say the only way to test fuel consuption is on dynos ect to get real figures.
do you think my figures are a good enough indication of what our car is using?
and should i use theese figures as fair dinkum fuel economy.
my agenda behind this post will be revealed in 4 tanks of petrol lol.
cheers.
and if your wondering wtf is a lacetti.
viva the revolution lol.