In a word: sh.t-house :)

Most XF's don't do much better than around 12l/100km on the highway, and
that's for well tuned EFI visions. Carburetted versions like yours most
likely is are slightly worse. If it's an auto, add around 20% on top of that
again.

Expect to be in the 14's or so for an auto carburetted 4.1 on a highway
cruise, provided you're not going to load it up.

Just out of curiosity, how good is your ute, and how much did you pay for
it?

Regards,
Noddy.