If the whole objective is fuel economy the Mazda 6 will probably not be close to the Alfa 1.4.
This site might help for cars that can meet US requirement - frequently LOWER MPGs than what can and will be sold elsewhere. You'll need to know what the specs are on that Mazda for the country it was originally destined and made for.
Here's a link that may help you decide??
http://www.fueleconomy.gov/feg/findacar.htm Been a long time since there were many Alfas in the US or at least seen by me locally. A rough guess is that the Mazda would be less on maintenance and repair than the Alfa but in the used market a lot more needs to be considered.
Good luck
T
_________________________________________
Tom Greenleaf - MetroWest, Boston