Just bought a 2011 Sonata and I drove ALL the cars in it's class. The major factor in me buying it was that I did the same loop on cruise at the same speed in all the cars and the Sonata showed the best mileage by a good margin, according to the MPG average computer.

The MPG average from the computer (on my car) shows 38-39+ for a full tank, but ACTUAL is 34.4 to 34.6 mpg.

588.5 miles/17.0 gallons = 34.617 mpg

First, it's just an estimate. Second, you have to reset it every time you fill up.

I have 2 Hyundai Corp cars and both computers are very cloce to manual calculation.

The computer on my 2006 Tucson 2.7L V6 averages a difference of +0.1 MPG to the manual MPG calculation.
It ranges from a high of +1.8 over manual to a low of -1.2 under manual.

The computer on my 2006 Kia Sedona 3.8L V6 averages a difference of -0.2 MPG to the manual MPG calculation.
It ranges from a high of +0.8 over manual to a low of -1.1 under manual.

Again, you must reset the MPG computer after every fill-up. Otherwise, the numbers won't be accurate. And you have to be sure to fill-up every time to get correct manual MPG numbers.

It resets itself upon fueling it.

After multiple tanks, it seems to be stuck on reading 3 mpg high.