It is possible that the 2020’s have this issue “improved”.

Hahah AlfaCrisis I can assure you it's got exactly the same issue.

After some fairly diligent experimentation I have some interesting findings. There are many data points here but I think there are a few crucial factors at play. We can all agree that the stated capacity of the fuel tank is larger than FCA officially states. The main reason I assert this is because the range does indeed fluctuate in correlation with my average fuel economy displayed in the driving data. I just filled up tonight, I was averaging 24.4 mpg at fill up and I got 392 miles of range displayed. If you divide that estimated range at full whether its in the 380s or 390s by 15.3, the mpg I would need to get that range is higher than what's on the computer by a few percent.

I think the tank capacity that the computer uses to calculate estimated range at full is around 16 gallons. 24.4mpg x 16 gallons = 390.4 miles. I will monitor this at every fill up and check the multiplier each time.

Second thing that threw me off recently was when I filled up two days ago only 3.57 gallons I went from one red bar and 16 miles of range up to 207 miles of range and 4 bars, after adding 3.5 GALLONS!! I was averaging 24.7 miles at the time of that small fill, and I had 283 miles racked up on that tank. 287 mi / 24.7 mpg = 11.62 gal.

If we assume 16 gallons to be the full tank, then I had 4.38 gallons remaining but the computer thinks I only had (16/24.7) 0.65 gallons remaining.

When I added 3.57 gallons to my estimated 4.38 gallons remaining then I would have had an estimated total of 7.95 gallons. This sounds about right as at that point the computer thought I had 8.38 gallons of fuel (207mi/24.7mpg). I continued to drive it and the range dropped exactly as expected as if I had about 8 gallons at that fill up.

I did not reset the trip computer at that point and racked up another 56 miles bringing my total to 343 miles when I filled up tonight. I was only able to put in exactly 10.3 gallons exactly. If we, again, assume that the tank is 16 gallons then I had 5.7 gallons at the time of fill-up. Assuming about 8 gallons at the last partial fill up, it is entirely possible to believe that I drove 56 miles using 2.35 gallons, which would come out to 23.8mpg, which is almost perfect because my average dropped from 24.7 to 24.4 over those 56 miles.

The data and subsequent calculations reveal a pretty good case for a 16 gallon tank, now it would be a matter of taking very precise notes and running experiments like this over and over again.

One thing is extremely clear however and that is the precipitous drop in range that the computer shows towards the end of the tank when in reality the estimated range at full seems to me to be very largely accurate. Why Alfa Romeo insisted on programming it this way is infuriating. What good is it for range to be accurate at the beginning of the tank and to be utterly useless at the end of the tank?!?