The way to measure is to divide the volume of gas pumped when you fill up by the miles driven - use the trip A or B to measure more accurately than the main read-out.
I found a drop-in K&N air filter (33-2320) gave an extra 1mpg better economy (I have a 2010 manual Aveo 1.4l). Low rolling-resistance tires (I brought Bridgestone) help a little + 0.5mpg.
I use a 'scan gauge e' (purchased via this forum) and it took a while to work out how to drive efficiently.