It's from an XFR, so heavier than an XJ (though maybe not much with all the extra kit and LWB), but I would say nowhere near. My driving was all long runs & averaged over about 25k miles.With the third birthday of my Autobiography ownership (and about the second birthday of the unicorn hunt), I actually did the arithmetic.
In three years I have averaged a measured miles per litre of 7.15. This is about 32.53 miles per gallon. In that time I have had it serviced annually, and additionally I have had four oil changes (inc. AdBlue). I don't do my own oily ragging, so it cost me £121.92 + £129.92 + £160.05 + £173.84 = £585.73. I didn't do any extra in the first year, hence at the beginning of year two I needed a throttle body: £1,146.28. Total diesel related cost: £1,732.01.
My diesel cost a total of £3,972.28 - an average cost of £1.63 per litre. Petrol is about 20p cheaper than diesel, so I will use a petrol price of £1.43 for the next bit.
If I had done the same mileage in a petrol car (and assuming all other things being equal - DVD players, dash cams, servicing, maps, AC regs, boot mats etc.), I would have needed to average 6.2763 miles per litre (about 28.5 miles per gallon) to break even with the all up cost of running a diesel.
So the question is for all you petrol X351 owners: what is your average fuel consumption, and does it beat 28.5 mpg (measured)?
It may be that the unicorn search is about to be abandoned, unless someone can tell me that it makes any sense at all (other then the sound, obviously)