Hi Wobble
First thing, how does your wife drive. If she dives like mine the adjustment in fuel burn could be justified.
Second, Only use good quality fuel (premium) I find that you can get good economy easily.
Third, be careful adding to fuel. depending on type of fuel system. may seem ok, although the long term damage can be expensive.
How many km do you have on the rig.
I run a Holden Colorado with the 4JJ1 turbo 3.0lt , the ute is 2.75 t on the road and average around 8.5km per 100.
approx 700km on around 65lt of fuel. Seems fine to me.
Good luck with it,
Paul