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Yeah, I suppose I might be with ya there on that one. There have been a couple times that I wondered why the nav was telling me to get somewhere a particular way. But I think those parts were local area versus the overall trip. The nav is clearly wrong at times on the local level.
That's why I use Google maps on my iPhone to take me the route I want - and let the Tesla nav "catch up". It eventually does, and I get to track energy usage.
I think it will be close, but you should be okay. You'll probably have to be at full or near full charge when you leave Wytheville. I recently went south on 81 to Chattanooga from the DC area. I left Wytheville with about 86% on my P100D and then stopped at Knoxville with something like 13%.