I'm sure that I'm going to be sounding like a broken record to some (broken record? WTF does that mean?), but the fuel economy indicator in the car is not an accurate or really terribly effective indicator of your actual fuel economy numbers. If you are legitimately concerned you should be tracking the ACTUAL fuel economy based upon fuel consumption versus miles traveled. In order to do this you must fill the tank completely every time and track usage for more that two consecutive tanks. Also, economy can vary significantly from tank-to-tank so it is always better to look at consumption over a long period of time rather than just one tank, or two.
The obvious things which can affect fuel economy is the style and type of driving, long periods of idling, and ancillary loads (like air conditioning use or significant electrical draws). Ambient temperatures can also have some effect, especially very cold weather (the air density decreases which will produce lower fuel economy), but for the most part warm weather works the same as hot weather so long as the A/C isn't switched on.
I myself have averaged just over 36 mpg total for the 4-1/2 months I've owned my car. Total mileage driven is about 7,800 miles so far, and there is an advantage to using the "ECON" button in the car, but it so drastically ruins the performance that the car is dissatisfying to drive (but I managed approximately 4 mpg better when using it). Everyone's results will vary, and I encourage anyone who's interested to find a method of tracking economy which works for them. Make an Excel spreadsheet, keep a notebook in the car, or use a website. I myself use gas buddy.com as they also provide local fuel pricing information based upon member submissions, which is helpful, and this website is free (I have no affiliation with them other than being a member and using the site for my own purposes).
Finally, I do use whatever is available as "Premium" unleaded fuel locally (usually 91 octane using the (R+M)/2 method), which not only offers a benefit to a high compression motor but also also should yield better fuel economy due to it not limiting the performance of the GDI engine.
I am attaching a screenshot of my current fuel usage tracking from the website that I use to show an example of the variance in my fuel economy. For what it's worth, my daily commute is (26) miles each way, mostly interstate highway, cruise control set at (65) mph with minimal stop-and-go. However, all of my non-commuting mileage is in town with heavy stop-and-go, and is typically about 1/4 of my overall driving. I suspect that if I could remove the in-town driving from my habits I could regularly return about (40) mpg from every tank.
