[not a hijack]
i've noticed a few times when coming to a stop while auto-stop has engaged: the instant MPG readout shows 150+ until your speedo hits 0mph, then your MPG drops to 0. However, you can still be moving very slowly, and your speed will still show 0mph (another side note, continuing to roll while at "0mph" will cause the car to turn on again - trying to prevent rolling)
so if this above case is true, then the MPG is determined by a digital readout of your speed, and of course fuel consumption. of course, like flipping a coin, there will be an average with a margin of error quite small, it's still interesting to think just how accurate the MPG readout is.
[/non-hijack]