It all depends how you drive the car. I got on a recent trip to Namibia 16l/100km ( 6.25 km/l) fully loaded,my speed was between 100km/h and 110km/h leading a convoy. This is with a standard 4.8. In the desert was another story , but got a respectable 3.2km/l in the dunes. On most of my open road trips my fuel average is between 18 and 19l/100km depending on the wind

When you go the chip route you also need to look at your exhaust system.