He measured those MPG values doing 2-way runs at each speed indicated. So they represent what your car would be getting at that speeds on a level road - it doesn't take into account the fuel used getting up to that speed, different weather conditions, etc.
His actual fuel log from EcoModder shows tanks in the low to mid 30's:
You CAN get tank to tank MPG in the 40's with an Aveo, but not likely with the automatic. You'd need the manual for that, very careful driving techniques and a MPG-friendly commute.
Your aveo most likely has an extended display in the center of the speedo cluster under a sticker above the odometer. You can hook up your own switch and I believe you will get a MPG display option.. There is a thread about it here, somewhere.