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Cool post! Thanks.

It must not be linear. I get an average of 4 miles/kwh and my 87% charge range is ~207 miles in my new 2021 Bolt. About 2-3 weeks ago, it used to predict ~225 at 87%. And when I bought it a month ago, it was ~259 at full charge. My current 87% range seems low but I guess it's the GOM. I will play with OBD2 to see if a cell went bad or something... but I'm guessing it's not linear (i.e. it's not simply 259*0.87 for SOC and rated milage nor is it simply 4*65*0.87 for average miles/kwh, 65kwh battery and SOC, etc...).

However, I do a lot of 70+ mph highway driving. My 4 miles/kwh comes from going downhill to work one way and uphill the other... all the while going 70+ mph mostly both ways. It somehow averages out to 4 miles/kwh though!
 

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It means you have ~59.5 kWh usable, like lots of others with the "66 kWh" pack.

Ask @Telek what is going on with these packs.
Lots of others? I can't seem to find any examples of others with this issue. Can you link some examples?

EDIT: This is interesting, but it's not 59.5 kwh which is quite a bit less than advertised for a 2021.

I will try the simple math proposed in that article using the infotainment's measure of spent kwh and difference in percentage from MyChevrolet app.
 

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From my reading of folk's posts, it appears that either the "66 kWh" packs are not 10% more capacity than the 2017-2018 packs, or the algorithms GM is using to estimate Ah, kWh, and GOM are screwed up.
I just noticed another example of what I see. Have a closer look at the WIND-WORKS link. Take a look at the GOM table for the 2020 Bolt.

You can see deviating estimates for lower miles/kwh values. For example:

GOM estimates that with
3.3 miles/kwh, the author has the range: low=217, mid=265, high=312
3.6 miles/kwh: low=192, mid=244, high=284
3.7 miles/kwh: low=227, mid=277, high=326

The GOM doesn't seem to be doing the simple linear calculation with our displayed average. It's doing something more complicated...
 

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I have a ‘20, and I am not seeing the same GOM estimates as that graph at all.

I just bought this ‘20 and had a ‘17 previously. The GOM is different between both, but both go towards the conservative side with the ‘20 super conservative.

I have only charged to full 3 times. The GOM prediction is 240, and my cumulative average is 4.1 miles per kWh. The furthest I have traveled during one of these full charges was 187 miles, and the car showed 31% remaining charge on the MyChevrolet app. I didn’t note the kWh used for that excursion, but the average was 4.1, so probably 45.6 kWh used, leaving me ~20.

With that data, I should be able to go another 82 miles on the 31%, for a projected 269 miles total for the battery pack @ 4.1mi/kWh.

The GOM occasionally adds a few miles within a few miles of initial driving.

All things considered, I would rather the GOM challenge me to beat it’s prediction than lull me into thinking I can up the speed.

Why mine or some 20-21’s read differently from others would probably need to be answered by “the ghost in the machine”.
If I had to guess, it's integrating something against the Efficiency(Distance) function to calculate its ranges. I haven't had a chance to play with the values in that efficiency vs distance graph yet (found somewhere in Energy menu in Infotainment system)!
 

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I will do some napkin math like the WIND-WORKS article to estimate my battery capacity (I've seen a youtuber do this too for calculating battery degradation). In the future, I will try OBD Fusion (no Android for me).

I am guessing that one needs to discharge 16 percentage points or more of battery to get a relatively accurate battery capacity measurement with the napkin math. This value can be found by analyzing the overall behavior of the round off error in the displayed SOC on MyChevrolet (the same can be done for kwh spent value, but if you spend 16%+ of your battery, this value is of less concern!).
 

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Part of the reason is that the SOC displayed value appears to be "sticky" - it will change in discrete 0.4 kWh increments. Obviously, that can lead to dramatic shifts in your capacity calculation if 16% = 10.2 kWh, 9.8 kWh (10.2-0.4), or 10.6 kWh (10.2+0.4).
The relative error of 10.2-0.4 and 10.2+0.4 to 10.2 is less than 4%. Not that massive at all.

All of your calculations at 16% SOC difference are within 3% relative error to 64. This is perfectly acceptable accuracy to me and I don't need to run down my battery a whole lot to get this estimate.

EDIT: That isn't to say that I disagree with you. Indeed, if you run down the battery more, your estimate will be even more accurate!
 
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