Trip Computer/Average Fuel Economy

Searched for a post on this issue but didn't find one. I have the Drivers Information Center/Trip Computer in my 2009 Aveo5 with the Average Fuel Economy display. Does anyone know how this thing works and if it is accurate? I reset it every time I fill up. I assume that is the correct thing to do. The owner's manual is not much help.

After I fill up for the past four or five tanks it always does this. Initially it displays 20.3 or 20.4 mpg for the first five miles. Then it jumps up to displaying 29.8mpg for another five miles. Finally, it settles on 26.9mpg and stays there without budging for the rest of the week no matter how I drive. When I fill up again and manually calculate my gas mileage only once out of the past four tanks was my mileage actually 27mpg. The other times my actual mileage was 23.6mpg, 30.5mpg, and 25.6. If I average the last four tanks I get 26.7mpg. Is that how the thing works? By averaging the number captured each time it is reset?

The owner's manual states, "This number is calculated based on the number of mpg recorded since the last time this menu item was reset." What the heck does that mean?

In other cars I have had the average mileage displayed was in real time and always changing. Is the Aveo display different?

Can anyone who has both a Scan Gauge and the Trip Computer report on whether they agree with each other?

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.

The fuel economy and distance to empty may vary significantly based on driving conditions, driving habits, and condition of the vehicle.