On Thu, 20 Jul 2017 12:59:17 -0400, Ed Pawlowski wrote:

On 7/20/2017 12:30 PM, trader_4 wrote:



If you're getting MPG on a long trip, you only fill the tank all the way when you start and
when you finish the trip. All the other gas stops you go by the meter reading.
Very little room for error if you write down the meter reading.

Agree. I was thinking of filling it full once.

Here's related question. Many new cars measure and show MPG.
How do they measure how much fuel is being used? It's not
via the tank gauge, because it can change almost instantly.
Do they have something that measure fuel flow directly? Or do they
do it by knowing how long the injectors are open and using that?
Anyone know?


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.
Roger clark forgets ONE major thing. The computer knows how much
FUEL is used - the energy density of the fuel doesn't enter into the
calculations at all - nor should it. You are measuring how much "fuel"
is burned - doesn't matter if it is gasoline or hooch. The computer
ASSUMES nothing. It calculates the volume by a combination of the
length of the injection pulse and the number of pulses, and the
distance travelled by the vss signal (which also provides data to the
speedo and odo)