Hi Paull,
My own new Ranger 3.0 posted a dramatic increase in mileage at around the 10,000 kms mark. Also, when you quote a mileage of x kms per tank, it leads me to believe you're not completing a tank to tank mileage test. The absolutely only way to calculate fuel mileage is to fill the tank, (without overfilling), reset the trip odo, put at least 300 kms on the vehicle, and refill the tank again and then compare the litres used against the kms travelled. Using the fuel indicator on the dash is completely useless.