All XJs are equipped with eight-speed automatic transmissions and automatic engine start/stop to help lower fuel consumption, and the lightweight aluminum body also plays a role in providing reasonably good fuel economy for such a large sedan.
The base supercharged six is pegged at 18 miles per gallon city, 27 miles per gallon highway or 17/27 mpg for the long-wheelbase version. If you add all-wheel drive, whether you're in a short- or long-wheelbase car, you'll drop those figures to 16/24 mpg.
Opt for the XJ Supercharged or its XJL counterpart, and the 470-hp V-8 returns mileage of 15/23 mpg, whether in short- or long-wheelbase form.
Fuel economy ratings for the top-performance XJR come in at the same place the 470-hp XJ Supercharged versions do.
The XJ's start/stop system one of the happier takes on this technology in the automotive universe, cycling on and off with a barely perceptible quiver--although you'll definitely notice the sudden absence of the baritone exhaust note in V-8 versions.