Respectfully, I'm not misinformed about sub-zero reserve. I can personally attest that there IS a 10-20 reserve since we had that happen before in FL in November. Here's a pic where we, too have driven at highway speeds for miles at sub-zero range on the gauge. You can see the SC stop on the display.
This trip from Charlotte was entirely an elevation DECREASE.
It was set to rated not ideal miles. "Ideal range" is just where unicorns live.
Temperature was 85. Sunroof closed. No leadfooting. The graph showed plenty of miles at arrival, the whole way... except for the last 50 miles when it was like someone poked a hole in the gas tank.
Highway traffic cruise set at 70, which we've done for thousands of miles. Uses more power than 60 but not THAT much. 60 with glass closed yields best range on flat roads.
40MPH speed taper came in the last 30-40 miles, which contains the slow driving part of the trip and where the yellow went to red far faster than the odometer.
Pack in September 2014 charged to 266 miles. Once. Now only charges to 253-259.
Algorithms Alchemy and other magic aside, seems to me that the pack is failing but they saw no errors and say the pack is fine, even though it went to zero like there was a hole in the bottom of the car and we were towing a truck and it shut down hard at 4 miles. No reserve capacity.
So did you attempt to utilize such a "reserve" this time or not? Again, if the car cut off at or below 0 miles, then everything is fine.
At 40 MPH that's 1.5 minutes per mile. If the A/C is on that's using about 2-4kW, so could be using a full 100 Wh/mile at that speed. At 60 MPH the HVAC would account for up to ~67 Wh/mi since it's usage per mile is only spread over 1 minute. So while running the HVAC you would be seeing rated miles drop *faster* at 40 MPH than you would have at a higher speed. If the battery cooling kicked in (very possible) then this would happen even with the cabin A/C off. Driving 40 MPH is probably a big part of the problem.
There is no reserve. The "reserve" people have experienced is simply a miscalibration that underestimated the initial actual pack capacity vs real world measurements and allowed usage down to the true bottom of the SoC. Once that calibration is accurate (after a few brushes into the < 10% SoC range) then that "reserve" disappears and is just essentially shifted back to where it's supposed to be as part of the rated miles.
Also, by draining the pack down to cut off is horrible for degredation, especially if the pack sits for hours in the heat or cold at this level. The photo above shows the limiter at something like 70kW... that's ridiculous. That's an amperage limit of about 80%C, which by my estimates put's the pack voltage down in the 2.xV per cell area resting. That's super low for the Model S pack, which I've never seen below about 3.1V per cell resting on my own vehicles at near 0% on the meter. So, any range loss you experience from running the battery to the ground... well, that's to be expected and isn't anyone else's fault.