I wrote a long explanation about the inaccuracies of the mileage...and then my computer froze and I had to restart. So I'll summarize;
The V8's(either 4.0 or 4.2) honestly get about 16mpg in city and a modest 25 highway
The V6's get only a little more on the city at about 19mpg but get a much nicer highway mileage at about 33 mpg(HONESTLY).

 

I think my car is an absolute cow, and I wish I could go back and buy the 4.0 to get that extra something, although I've been getting great gas mileage and at 106K miles I've only had a couple of bumps along the way.

 

Ive driven 3.0s for around 10 or 15 days total since ive had my R ( when i leave it to get serviced for a week at a time in another city) and have found not much of an increase in fuel economy during either city or highway driving. If you get the 3.0 with the newer 6 speed transmission it will most likely have plenty of power at highway speeds as the transmission works quickly to keep the engine in its power band. Off the line it is a little sluggish though. The main cost diffrence occurs at aquisition. hope this helps you!

 

Same here my wife and I drove seperate cars equal distances. She was driving our Honda Element inline 4, while I was driving the Jaguar S-type 4.2.According to the onboard Jag computer I was averaging close to 34 highway miles and everytime we stopped to fill up the Honda would take twice the amount of gas as opposed to the amount of gas theJaguar would take. The 4.2 rode over 350 miles on 3/4 of a tank.

 

so far the 4.2 has been excellent for fuel mileage hope the 5.o litre will be just as good.. of course with the help of the 6 ZF
the difference between the V6 and V8 was a $2000 difference when ordering the car.. new