Palbay,
Autoweek had this piece which has 22 mpg combined, implies the estimate came from BMW ( claimed).
Producing 400 hp at 5500 rpm along with 450 lb-ft of torque between 1800 rpm and 4500 rpm, the V8 delivers 50 more hp and 100 lb-ft of additional torque compared with the X5's naturally aspirated 4.8-liter V8. That's enough power to launch the X6 from 0 to 60 mph in just 5.3 seconds--faster than a 650i automatic--while giving it a top speed limited to 150 mph and a claimed combined fuel economy of 22 mpg.
http://www.autoweek.com/apps/pbcs.dl...1581/1008/FREE
of course still doesn't tell us if this was for US version or export version.