On Thursday, July 20, 2017 at 2:29:01 PM UTC-4, 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...-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.


Why is it hogwash? They seem to be saying the same thing you are,
that the energy content means that what you see as MPG will
vary because of it. If you have gas with higher energy content,
then you'll see 25 mpg. If it's lower energy content, you'll
see 23 MPG. How much the energy content varies, when it's
labeled as the same product, eg reg unleaded, IDK. It would
obviously vary depending on the amount of ethanol added.