I know lots of other threads touch on this subject but I can't find a concise answer. Does anyone actually know how remaining miles is calculated? Is it simply based on pack voltage?

I would think so. That's certainly how we measure longevity of implantable defibrillators. Batteries have a very predictable discharge curve, especially when little cell variations are averaged out across a whole pack. It would make sense to map voltage to that curve to get "percentage" charge.

That makes sense. Where it get's a little fuzzy is with my 60D (75 battery). Delivered (8 months ago) range was about 222 and now it 215. The only thing I can think of is that the "mapping" algorithm changed. Let's say 100% on the 60D is 80% on the 75D battery. For simplicity's sake lets say that is 8 volts out of 10 volts (I know it's really closer to 4) . Assuming my drop is due to voltage...this means at delivery had 8 volts and now I have ~7.8 volts. It could be true that the 75D battery may have degraded a minor amount, so lets say it went from 10 to 9.8. That is still well above 8. So I don't see how the 9.8 can factor in to the remaining range calculation. My car can't even charge to 100% of the 75 battery so it wouldn't know that. Charging to 8 volts should easily be attainable. So still puzzled as to why the drop 222 to 215. Please note that I don't care about range loss, I am just trying to figure out how the system is working...

The car also measures the amount of current in and out of the battery. The voltages mainly act as limits to prevent damaging the battery by over- or under-charging it. A degraded battery will allow less amp-hours of current to flow before hitting the voltage limit.

Unlike lead acid batteries where resting voltage is a good proxy for state of charge (SOC), for lithium ion batteries voltage is not a great proxy. The resting voltage is fairly constant between SOC ~10-95%. So knowing the voltage for an individual cell could mean you are about empty, or about full, or somewhere in between. Most battery management systems use a shunt to measure current over time (amp hours) in order to estimate state of charge. If you have a cell with a 100 Ah capacity and you measured removing 25 Ah, you should have 75Ah remaining. The algorithms for range estimates probably use Ah.