Average MPG Indicator On New GM Cars ?

Hello,
Have a new Buick, but imagine it's similar for all new GM cars.
Re the display that shows the approx. mpg:
There is a reset button that one can push.
Is the mpg computed from the push of the button, continuously, irrespective of how long the trip is ? e.g., the mpg is a "true" average for the entire trip.
Or, does it continuously re-compute the average mpg, perhaps every few seconds or so, and updates accordingly (for that few second period) ?
Thanks, Bob
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Hello:
The mileage is computed in real time, years ago. Before Mass Air flow Sensors, the engine vacuum was used as the main input. However with the advent of the MAF sensor controlled systems, the MAF and engine vacuum, as well as throttle position sensor are used.
This is also based on the fuel in the tank, for the range readout.
I hope this helps.
RK
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