This seems to be an unusual experience, based on people's reports here. Have you asked whether the GOM is fixable, or needs to be replaced?
So, the Kona i have is a 39.2 KWh one, not the 64KWh. The range estimates for this variant usually starts at around 330KM's and if driven efficiently, it starts showing higher values on subsequent drive-recharge cycles.
But the range estimate max's out at 393km (with efficiency calculated as 10KM/Kwh), it is almost like a hard limit, no matter how efficiently i drive it.
Now my consumption is around 11-12km/KWh (around 7miles/KWh) or in KWH/100KM terms it is equivalent to around 8-8.5KWh/100KM (around 13KWh/100miles). So, that usually gives me a range of around 470KM's (292miles) almost regularly. I have even tested it on a long drive, where i managed 500+KM on a single charge.
(Yes, it is better than the WLTP rating

, but i have been able to manage it almost consistently for quite some time so can vouch for it.)
This is after almost a year's worth of driving, BMS upgrades, clearing out the memory etc - almost all that, I and the service center, could think of. Hence the conclusion that the range estimation has an upper hard limit of 393 km or basically 10KM/KWh. So, now i do not use the GOM but instead rely on reverse calculating using the Efficiency visible on the dash.
The SoC on the other hand precisely tracks what's available (in coulombs to be pedantic) according to all the OBD measurements I've done over the years.
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True. This would be the most precise calculation for the range. The SOC on the display is never as accurate as from the OBD.
On this recent trip I've used OBD data to log cumulative net draw from the battery by kms, the blue line. At the end it works out to 13.79 kWh/100km but the dash says it's 13.4.
The red line is the blue line but minus the power going to the 12V system via the LDC, basically what HV power is needed to keep the car alive outside of AC and heating, neither of which were used here. On the "Electricity Usage" screen that's the column labeled "Electronics", usually showing about 200 watts.
I just picked that factor arbitrarily but the result of 13.34 does suggest that the dash reading may be ignoring that particular use of energy.
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I came to that same conclusion - the LV battery(12V) consumption may have been missed out.
The efficiency displayed on the dash vs the calculated efficiency using Odometer(kms) and SOC from OBD, had a shortfall of almost the same amount as the "Electronics" consumption calculated over that duration.
I had noticed the following on a night drive (cool climate) - only the blower was running(without the AC), so basically no consumption showed up in the "Climate" section. And almost all lights including the fog lamp and the internal ones were on, so the consumption of "Electronics" was almost at 0.45KW. On that trip the shortfall of the efficiency shown on dash was higher and almost matching with the higher "Electronics" consumption.