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So going on my 1st fully charged road trip today. Charging complete shows 307 miles and I haven't yet hit 1000 miles in the car- what are others getting? Not a big deal, but at this point I'm feeling death by 1000 pinpricks
There's a good summary of a conversation with a Tesla technician on page 11 of that thread.
TL;DR, it's slight errors in the state of charge calculation likely caused by shallow discharge cycles (not enough information to return an accurate result) and not an actual problem with the car. Doing deeper charge cycles will restore the accuracy of the state of charge display.
Link from @Big Earl is spot on. I too experienced a 1% (3 mi) loss in range with about 1,3xx miles on the odometer with my TM3P with PU (P3D+), but it's acceptable. No battery is going to stay like new with 310 miles rated range for a long time.