Did you guys consider the weight of the gas in your completely full tanks when trying to figure out what is going on?
Anyway, I can typically get more kilometers per hour when my tank is half full as well. We all have to remember that our stock gas tank is at the very back of the vehicle. The last time I checked I was able to get 418km on a full tank of gas. Then again that was about six months ago and I have not really had to put more gas in the car due to me not driving it very much.
I am not sure what the driving habits are for most drivers there in SA, however here in the states a lot of people put there cars in neutral when either going down a hill, waiting at a traffic light, or coming to a stop.