I don't have the Driver Information Center, but with my Scan Gauge ii, the AVG gets updated every couple seconds (the Aveo's may update a little slower). It really is a running tally of everytime you drove the car since you last reset it, by pressing the button. (That's what that wordy description means.)

You may not have an "Instant MPG" on your car. My 2011 Silverado with the 4.8 DOES NOT, whereas the 5.3 DOES.