Since you've not had any answers yet I'll put in what I know, but not sure how relevant it is.

I used to have a 3.2 XJ40 and now have a 3.2 X308 and to be honest, the economy difference is zero. The only difference is that the V8 is smoother and has a bit more grunt.

If you go for an early XJ40 though, upto 1989ish I think, the only engines available are the old 2.9 and 3.6. Early XJ40's will have the gremlin-riddles digital dash, while the late ones from 1993 onward will have almost all the X300 improvements and goodies.

Like I said, not much info of use there for a 4.0, but if the developments in the 3.2 are anything to go by, the economy should be similar and in a realistic world probably around 20mpg.

Let us know if/when you find one

 

I went from a 1992 3.2 xj40 to a 1997 4.0 x300 and found the x300 to be slightly better on fuel than the earlier car also a lot smoother and noticeably quicker although it is a bigger engine of course.

 

Ok, well here is my 2p worth.

I had a 2.9 XJ, went to the 4.0l X300, fuel economy was better as were the general maintenance bills. Not quite the similarity as your question. Try this link:-
http://www.wyn-thomas.co.uk/frame8.htm

 

i have to say that what ever engine you choose the biggest factor on MPG is HOW YOU DRIVE IT! so i would buy the best car you can on your available budget, and worry less about what engine size it has/ what year it is. and more on what condition it is, and the condition of the rest f the car, as that's what will eat your money. keeping it running.
this excludes the V12 though of course. that's a different beast altogether! and cant really be compared in the same breath as any 6cyl.

 

i actually run both a aj6 4ltr xj6 and a aj16 4ltr x300 and fuel economy is hardly noticable but the aj16 seems to be just a little more smooth and refined power. i stay up in aberdeenshire were the roads are shhhh... and i can average about 25/27 on local runs and 33 plus on a good long run. compared to my last xj5 3.2 which was 18 and 27 respectivly. but like one other said its how you drive it as i can put £20 and it will last a day or it will last 2 or 3 days