this is the 3rd mention of fuelly.com:
http://www.fuelly.com/car/mazda/6

why don't you check that out instead of asking here?

summary/average: they go between 11-13km/l and chances of heavy traffic jams in the US would be lesser compared to us.

expect 8-10 km/L

heck, didn't a mazda 3 posted his FC? considering a 6 is much heavier, further reduce maybe 2-3 km/l to be around 10 km/l maybe?
10 km/l is about 10l/100km, half of what was shown on the meter i guess

The reason why everyone else criticized on this thread, is that when you're buying a heavy D segment car, FC should be the least of your concern considering you're paying a huge premium for the car already.
If you're really that concerned, and the Mazda 6 seems to be in fact not a great performer in FC, consider another car then. (like the turbo charged ones that are in smaller cc that could probably be more frugal in FC)

=edit=
corrected previous mistake on mazda3

This post has been edited by Quazacolt: Oct 3 2014, 04:39 PM