The rated range is the 100% charge estimated by the car after each charging session divided by the rated wH/mi. Thus if a S/W update changes the displayed Wh/mi number the displayed range may change even though the battery hasn't. Of course Telsa may change the software's definition of empty and full and thus the displayed battery capacity may change even the the battery hasn't. In any case, to remove one additional layer of uncertainty I track the estimated capacity of the battery in kWh rather than the mileage. It is possible to estimate the battery usable capacity from the TeslaFi reports. Simply divide the kWh added in a charge by the charge difference in percent and multiply by 100. The uncertainty in such a capacity estimate is approximately 40 divided by the difference in percent. Thus if a charge from 51 to 76% added 25 kWh the estimated capacity is 100*25/(76 - 51) = 100 kWh ± 40/25 = 100 ± 1.6 kWh. The following picture shows the history for my X. Note that the trend line was computed taking the uncertainties of the estimates into account.
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Though the data so far make it appear that battery capacity has actually increased slightly (there is a 93% chance that there is actually no increase or decrease at all) since I have had the car I expect the trend to turn downward fairly soon simply because that's what happens with most cars.