How pessimistic is the Mi/kWh display on the dash?

[GoHawks]

Something new I noticed on my commute today - differing displays and calculations for mi/kWh. I do not use the GOM for anything and usually do the math when driving long distances. I have noticed using my ODBII device the dash display is generally pessimistic on what it shows once I do the calculation post trip. I attributed it to rounding but why do different screens in the truck show different results? Has anyone else done a deeper dive into specifics on this using data for measurements? If so, please provide results. That extra ~.1 or .15 mi/kHw might help change charging locations when traveling.

 

[TaxmanHog]

This has always been an issue

 

[JRT]

If you are traveling relatively flat interstate at 70 mph with no head wind, j use 2 m/kwh always. After just under 15k miles I never have seen a miracle change like others post.

 

[Jseis]

This subject gives comic relief given the five hashtag marks gauging fuel tank volume many of us grew up with. I drove 52 miles with my Bug on “R” one dark eve when all stations were closed. To this day I’ve no idea of volume but I definitely used all of “ R”.

 

[Pioneer74]

It's been like this since the beginning. Nobody knows why, except that math is hard.

 

[BarryGoyette]

For me, I generally ignore the GOM. I plug my destination into Apple Maps, and it gives me a destination SOC that tends to increase slightly during the trip. I find that comforting.. and so far it’s otherwise exceedingly accurate.

 

[PatchesPal]

I have always been curious what time or distance the mi/kWh is based on?

For example, is it the average of the last 5 miles or the last 5 minutes?
Or is it the average since the trip started?

 

[RocketGhost]

I have always been curious what time or distance the mi/kWh is based on?

For example, is it the average of the last 5 miles or the last 5 minutes?
Or is it the average since the trip started?

It's the average for the trip. It wouldn't make since if what it showed for "this trip" or "trip 1" wasn't actually for "this trip" or "trip 1."

 

[chl]

Like any computation, the value is based on the data input to make the calculation.

We can assume the mileage number is pretty accurate based on wheel rotation.

How well the kWh's are measured is a question mark.

So the time (hours or fraction thereof) probably pretty accurate.
But how is the power (kW) determined?
The terminal voltage of the HV battery x the measured current we assume.
But the terminal voltage will decline slightly as the battery discharges maybe such a small amount it is within the margin of error of the measurement tool.
And the current will fluctuate as more or less is used, so it has to be summed/sampled and digitized which introduces some error as well.

Without knowing the details of how the mi/kWh number is actually calculated, hard to say how accurate it is - so let's call it a ball-park number and be done with it!

 

[chl]

Electricity meters operate by continuously measuring the instantaneous voltage (volts) and current (amperes) to give energy used (in joules or kilowatt-hours, etc.).

The standard electric power meter is a clock-like device driven by the electricity moving through it. As the home draws current from the power lines, a set of small gears inside the meter move. The number of revolutions is recorded by the dials that you can see on the face of the meter.

A mechanical electric meter has two conductor coils that create magnetic fields. One coil is impacted by the voltage going across the conductor, and the other is affected by the current going across the conductor. The magnetic fields generated by the interaction of these coils then turn a thin aluminum disc at a controlled speed proportional to the amount of electricity consumed.

The spinning of the disc moves the dials that indicate the total electricity consumed in kilowatt-hours.

Now I haven't seen one of those in my Lightning...so how does the lightning do it?

It is some kind of non-analog digital approximation.

 

[RickLightning]

Interesting how the majority of the responses don't seem to understand the question.

Pictures 1 and 3, and likewise 2 and 4, display the same data. Yet, the miles per kilowatt show for the exact same data is one-tenth different.

The answer is:

Bob Franklin is the person that wrote the code for pics 1 and 2. John Heratio wrote the code for 3 and 4. Bob went to school in New England, and is really smart. John went to school in the south... He's not good at math...

 

[chl]

A number of sources of imprecision and estimation in a digital energy meter:

How accurate are the current and voltage measurements?
What is the frequency of sampling - will introduce some sampling error?
How is the calculation done in the microprocessor - is there rounding off?
(Note: the "Battery" in the drawing is the battery powering the meter, not the battery being measured)

 

[mb0220]

Interesting how the majority of the responses don't seem to understand the question.

Pictures 1 and 3, and likewise 2 and 4, display the same data. Yet, the miles per kilowatt show for the exact same data is one-tenth different.

The answer is:

Bob Franklin is the person that wrote the code for pics 1 and 2. John Heratio wrote the code for 3 and 4. Bob went to school in New England, and is really smart. John went to school in the south... He's not good at math...

I like your "Bob" and "John" analogy. As an IT professional, my guess is that both screens are using the same data point, but for the display value the center screen Bob is rounding (1.87 becomes 1.9) while in the driver display John is truncating (1.87 becomes 1.8).

Still very annoying to those of us with moderate to severe OCD.

 

[chl]

Interesting how the majority of the responses don't seem to understand the question.

Pictures 1 and 3, and likewise 2 and 4, display the same data. Yet, the miles per kilowatt show for the exact same data is one-tenth different.

The answer is:

Bob Franklin is the person that wrote the code for pics 1 and 2. John Heratio wrote the code for 3 and 4. Bob went to school in New England, and is really smart. John went to school in the south... He's not good at math...

OK, so the questions were:

1) "why do different screens in the truck show different results?"

and

2) "Has anyone else done a deeper dive into specifics on this using data for measurements?"

Answer to 1): One screen is "This Trip" and it is a 41.1 mile trip, while the other is for "Trip 1" which 1833.6 miles. Over longer distances the driving style will be different from shorter trips is one possible explanation, another is the short trip is a smaller sample size so maybe less accurate than the longer trip (but only by a hair different).

Answer to 2): I think I beat that question to death in my resplonceanations.

 

[hturnerfamily]

your long-term trip meter, like TRIP 2, especially if a lot of miles, will generally give you a more average idea of your driving characteristics, range-wise/mi-kwh...
but, I've noticed that the GOM is generally much less optimistic about my overall range, at the outset, than I know I can normally achieve...

if I'm setting out on a long trip, and I've only drove around town for a few miles over the last several days, it might tell me my Pro can 'go' 290 miles...wow! Reality is, though, that it is 'assuming' that I will have that same 2.9 mi/kwh estimation during the whole upcoming trip. It tends to settle back down, though, after some miles into the trip...

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