I recently read a post on ways of improving fuel mileage on Chev Aveo's. As per the post, I replaced my air cleaner (which was a fairly new OE GM air cleaner) with a K&N filter and a set of new spark plugs. The old stock air cleaner looked okay, but the spark plugs were BADLY eroded (about 70k on it at the time). Mileage jumped from about 350km per tank (mostly highway miles) to over 500km per tank (sometimes over 550km).
My mileage is starting to drop again to around 400 to 450km per tank. I cleaned the K&N air cleaner (as per instructions and replaced filter oil) and checked the plugs. Some slight erosion, but still in spec (0.044 inch gap).
My only thoughts are that mileage would be dropping (now down about 20%) due to the change in temperature - I did the tune up in summer when the temp was about 25C and now its about 5C.