On Thu, 20 Jul 2017 13:28:54 -0500, dpb wrote:

On 07/20/2017 11:59 AM, Ed Pawlowski wrote:
...

Based on my checking over a tankful and what the meter says, they just
take an optimistic guess. It is always higher, but there seems to be an
explanation

https://www.edmunds.com/fuel-economy...s-fibbing.html

Roger Clark, senior manager of GM's energy center, explains that the
fuel economy gauge makes a calculation by counting the number and
duration of pulses made by the fuel injectors as they squirt gasoline
into the combustion chambers of the engine. The onboard computer system
divides the distance the car travels by this estimated fuel consumption.

Clark says the gauge is "dead nuts accurate" — if you consider all the
variables at work during driving, including temperature, driving
conditions and driving style. The biggest fluctuation occurs because
ethanol, which is blended with gasoline in varying amounts, contains
less energy.

"When you fill up, you are paying for a gallon of gas, but the energy in
that gas varies significantly," Clark says. This means that while the
car's computer assumes the gasoline is providing energy to drive a
certain distance, the fuel might have less energy and not propel the car
as far.

I think that's hogwash, too. The mpg is volume/distance; the
computation is independent of the energy content; simply a higher
consumption rate/lower mpg value will be computed.

_Unless_ they don't use actual odometer reading and distance is imputed
from other measurements at least one of which is based on engine
efficiency under given conditions.

If they base their fuel consumption on exhaust gas analysis, rather
than actual fuel measurement he is correct. I believe that is how the
EPA rest works (and why it is NEVER accurate)
I don't have inside info on the precise algorithm, I'd always figured it
was, in fact, simply the instantaneous volume*t integrated and the
accumulated is simply the ratio of the two sums (integrals) of fuel
volume over total distance since totals were last reset.

The Buick longer-term values have always worked out to be pretty-much
identical to logbook records on long trips for several different models
from the old LeSabres to the (relatively) new Enclave and Lucerne even
though they're now getting long in the tooth by car sales standards...