Its quoted at 42mpg I believe, but in reality I dont think an XF S daily drive will get anywhere near it.
As it is so large and has a decent ride quality with good soundproofing, you do have to watch the speedo. You can get well over the posted limits and not realise it. It just doesnt feel as fast as it is. At low speeds it feels like you could get out and walk quicker. Its just built for getting you points in a 30 or 40 zone.
get rid!