2012 Malibu LT 2.4L w/sudded drop in fuel economy.
 

Until about 100kmi on the odometer, the vehicle got 24mpg city consistently, then suddenly dropped to about 17mpg. I changed the plugs and air filter, and it bounced back to just under 24mpg for about 2 fill ups, and now I'm back to around 17mpg. Seems to run great, and no engine lights. What can it be?