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80% charge went down in range mileage

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This is quite normal, as a matter of fact, you will see your range drop about 3-4% in the first year of ownership. After that the drop seems to be much less. Do a search on this, there are a few large threads on this subject.
In addition, you'll find that the car continually adjusts / calibrates. The number of rated miles it displays isn't precise, because it's based upon voltage levels in the battery pack. My car showed 264 miles maximum on a 100% charge the other day until my wife drove it on a trip; she took it down to 25 miles, then charged back to 100% at the supercharger and it showed 265.

So you'll see max mileage occasionally fluctuate a couple of miles or so.