On Fri, 21 Jul 2017 08:35:10 -0600,
rbowman wrote:

For starters, I am not talking about a dashboard display provided by the
manufacturer. As I've said repeatedly I use a ScanGauge II plugged into
the OBD II bus. How accurate and repeatable is the ECU? Tach? That would
depend on the smoothing algorithm for the display. Counting the ignition
pulses is a no brainer. Distance? The unit can be calibrated for the
rolling circumference of the tire. Variability on tire temperature and
pressure? Sure. Injector timing? Temperature? How good is the sensor?

I think you're answering a different question than is meaningful to most
people simply because most people don't have a ScanGauge II to plug into
the OBDII port.

Most people have a tripmeter reading and a gas pumpmeter reading.
Where they fill the tank and reset the tripmeter before driving away.

I can't find any reliable source that says what the accuracy or
repeatability of that mom-and-pop tripmeter/pumpmeter calculation, but
basic logic dictates that the errors compound such that there is likely
(IMHO) no way to get anywhere near decimal-point accuracy, and worse,
probably plus or minus 1 mpg is the closest anyone can get in terms of
repeatability and precision.

So, someone's 20 mpg is really somewhere in between 19 mpg and 21 mpg.