At the moment i'm also getting terrible fuel economy. To give you an idea, The last trip I went on over Christmas to Coral Bay, I was lucky if I got 300kms per half tank (Each half).
The last tank full that I put through it on the way home including the sub tank, I only got 724kms out of the 120 litres. I'm not sure what that works out to be for L/100kms - Not sure I even want to know LOL
The car is mechanically stock. It's running standard tyres etc and has the Nissan bullbar and snorkel from factory. Just before Christmas I put a roof rack on it for extra storage space. I knew that this would affect the fuel economy even though I didnt have very much on the roof (Tent, 2 fold up chairs, Mattress and Gazebo) Nothing was stacked up ontop of anything. They were all laying side by side as I bought the full length Ali Rack.
On the way back from Monkey Mia I cracked the *****s with the fuel economy and put everything from the roof back into the car. This made no difference either.
Originally I put the crap fuel economy whilst touring down to a few things; Excess heat outside, Odd head wind, Extra stuff/gear in the car (weight), Using aircon (sometimes), Crap fuel etc.
But since returning the economy hasnt got much better. (I had done the basic's whilst I was away - checking air filter or blockages, tyre pressure etc)
I'm now in the process of trying to track down why the economy has turned to ****.
I've purchased a scanguage II and i'm in the process of calibrating it with a few tank fulls of fuel. The scanguage shows no errors in the cars ECU either. Tomorrow new oil and filter go in along with a new air filter. (FYI I always change the air filter every 5000kms with the oil and oil filter and my mechanic checks the MAF at each service too, this was all done 2 days before I departed for the holiday)
I'll also be keeping logs of the fuel economy with notes on the type of driving style and with aircon or without etc
This weekend I may be travelling to Albany, which is about 800-100kms return trip. I'll be packing the car as light as possible and removing the roof rack for this trip and see how it goes.
I'll let you know how I go