15-20 mpg seems awfully low in my experience. We have an average of 26mpg on our 2000 XJ8 Vanden Plas which we have put 50,000 miles on since new, and are averaging 24mpg in our 2003 XJ8 Supercharged model Vanden Plas which now has close to 6,000 miles on her. Granted I would say that around 80% of the time on both vehicles it is highway miles, but for 2 and a quarter tons of metal, it seems not bad. I would have expected the S-Type's to weigh less than the XJ8's and get better economy!