To all who are worried:
May I suggest that you do some real MEASURED and CALCULATED fuel economy tests before you jump to the conclusion you have a problem.
For example, what you "think" is 15-18mpg may actually be closer to 18-20, and that would not be totally out of an expected performance range for a typical "stock" XJ.
Also, on a typical XJ, with a 20 gallon tank, it probably reads "E" when there is still 2-3 gallons left in the tank. Gas guages tend to be conservative--there is nothing more agravating than to run out of gas when the guage says 1/8th of a tank is left!
Thus, the typical owner is only actually using about 18 gallons of fuel when he reaches "E". At 18 mpg average fuel economy (combined city/highway), that equates to a range of 324 miles.
I would also bet most of us don't actually wait until the needle on the guage is on "E" before we fill up. So that means we are really refilling when there is maybe 3 or 4 gallons left, not 2, thus making the total refill closer to 16 gallons.
Sixteen gallons at 18 mpg is a range of 288 miles. That sounds like the kind of range many drivers of XJ's report, and yet think they have problems.