Guess I can use this to note the things I've found about driving the car. My car for reference is a base GT with RWD. In Eco Mode the car is averaging between 24-25 MPG in the city, however I believe the higher city MPG has more to do with the 45 mph speed limit on most of our surface streets. As the car (On the 19 inch rims and stock summer tires) shifts into 8th at 47mph, the computer calculation at this speed is about 38MPG. At 5mph intervals the car loses roughly 2MPG. So at 65mph highway cruising speeds I've gotten the car to do around 30MPG (I've seen it as high as 32 in some cases.)
Of course if you keep the car in sport mode it keeps the car a gear lower in most cases, I find that in sport mode my average MPG is around what the EPA estimated is. High teens for city and mid-upper 20s on the highway. In my experience using the onboard computer and doing the math between fill ups I'd say the computer is within margin most of the time, maybe half an MPG tops.