The recent Cape trip I think I used around 23ltrs per 100klm towing a fully laden camper. Its only a 6 cyclinder.
As mentioned knowing the distance you can travel is the main objective.
But for the cost of the fuel, $2.40 per litre for petrol in Siesia, knowing what fuel stops are along the way and fuelling up where it is cheaper helps.
For the amount spent on the trip for fuel, it pals into significant, compared to the adventure and fun I had on the trip. Memories are priceless.
Mark