Is there something wrong with my pack, or is there a ~10% “buffer” not represented by the SOC% display?
There is nothing wrong. There is some buffer but it is not 10% (it's more like 3%). I am not sure exactly what your pack capacity should be for SR but it is probably something like:
220rmi * 219Wh/rmi = 48.2kWh
(rmi = rated mile)
There's a discussion on this page (read the 5-10 preceding posts as well
):
Empirical Trip Meter Wh/mi constants
The trip meter is not representative of your battery capacity. What you should look at is the charging screen for a charge cycle - say you add 150mi to your pack, you will see (if you switch to %) that that corresponds to 33kWh (32.85kWh = 150rmi*219Wh/rmi ).
So you can extrapolate that to 32.85kWh * 220rmi/150rmi = 48.2kWh (or 52.56kWh for the SR+)
But beyond that, there is some reserve energy (if the BMS/Soc estimation is working correctly) below 0 miles. It's 2-3kWh. So add that and you're at the ~54.5kWh.
The trip meter, for the SR+ (and probably the SR), on the other hand, apparently will show ~209Wh per
rated mile used (not per mile traveled!!!). (I've never verified this myself since I have a Performance, but see that other thread.) So for an SR you'd probably see it extrapolate to a discharge from 220rmi to 0rmi as 220rmi * 209Wh/rmi = 46kWh.
Note also a little subtlety: the EPA discharged the battery to dead for its tests (not to 0 rated miles). So actually the EPA rated miles aren't the same as the rated miles on the battery gauge I refer to above (the rated miles on the battery gauge above are slightly
lower energy content than the EPA rated miles, by approximately the % of reserve capacity)
If these numbers don't match your mental math on your next road trip, do check back in here with any corrections. (Obviously the trip meter does not take into account battery use when in Park, so be sure to avoid that state if you're doing math.)