The tyres are a real killer, I was getting 9.0-9.5L/100km with an 04 ZD30 stock and basically when I put 265/75/16 LT tyres that was when my fuel economy went to 10-10.5L/100km.
But this is because you increase the tyres overall diametre by 50mm and the weight increased by 4.5kgs (per tyre, this is a lot in terms of unsprung weight) from the extra size and extra plys for the LT tyre compared to passanger tyres.
I dont really notice much power drop but the fuel consumption speaks for itself. And L/100km is with the 0.03% factor measured for and equated in.