I have a 25t Petrol and just wondered what mileage people got. Mine seems to have gone up recently to about 12l/100km. (apologies to imperial friends). That's about 23.5mpg.
I used to get 7-9.
Ps I'm in Australia.

 

I get 25 mpg city and 35 mpg highways on mine. Of course I don't normally floor the gas pedal from stop to go. Be gentle on the gas pedal. Your mileage alao depends on passengers on board, the things and stuffs inside the trunk/boot (leave the bag and set of golf clubs at home if you're not going to the range or the golf course and the heavy luggages if you don't plan to travel). Your tire pressure should be correct. I do mine whatever MAX tire pressure stated on the tire (a lot of peeps on here would disagree but it's just me, that is why I get away more than the required MPGs ) Inspect the air filter, it should be clean. Use Premium gasoline/petrol

 

Just had a service, new plugs and air filter.
I read about the MAP sensor getting fouled up that can cause this so i checked it and it was clean.
FOMOCO part incidentally.

 

If you XF's engine is a Ford's EcoBoost that's a different story. Mine is an JLR's Ingenium. Good luck

 

I believe its an ingenium.

 

OK, I forgot something, maybe one of the bearings is shot or maybe more. Sometimes it makes noise sometimes it does not. To test this, park in a slightly inclined area and put the car in Neutral. Don't turn off the engine or else it'll go to P, Let go of the brake and see if the car rolls by itself. If it doesn't or roll a little then you've found your problem. Check the brakes as well if they're sticking

 

I think its just me. Varies between 8.5-10 mostly. Sometimes higher sometimes lower.