Weight and aerodynamic drag are the enemies of fuel consumption. As a rule of thumb, once you drive in excess of about 80kmh the issue of drag becomes quite significant...e.g. for a 25% increase in speed say from 80kmh ->100kmh, the drag increases by 50%. If you then increase your speed to 110kmh, the drag is 90% higher than at 80kmh! 
Loading up a roof rack, adding a bull bar just increases the drag.
The other enemy is weight...bull bar, dual battery, full size spare wheel(s) and sundry camping gear we tend to lug around... pretty soon the numbers add up.
If you want to test this, remove all the added stuff and do a couple  of hundred km at a steady 80kmh...I would not be surprised to see the fuel consumption reduce to below 10L/100km.
				
			 
			
		 
			
				
			
			
				MY99 RR P38 HSE 4.6 (Thor) gone  (to Tasmania)
2020 Subaru Impreza S ('SWMBO's  Express'  )
2023 Ineos Grenadier Trialmaster (diesel)
			
			
		 
	
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