Fuel economy's mostly an afterthought for anyone shopping the 2012 Jaguar XJ. Still it's worth pointing out that the XJ doesn't get exceptional fuel economy even though it has a lightweight aluminum body structure.
Without any options for diesel or hybrid models, the XJ lineup is at its best in the base short-wheelbase version. The EPA rates it at 16/23 mpg. Adding supercharging to the 5.0-liter V-8 drops fuel economy to 15/21 mpg. Meanwhile, the long-wheelbase edition is rated by the EPA at 15/22 mpg.