I think I need to give up on trusting the built-in mi / kWh metering for any of this.
I was able to find a nice straight level section of road today with near zero traffic for 8 miles that should have provided very consistent readings against the plots, but I can't make the numbers work out in any way that makes sense. While I have been able to produce results that fall inline with the curve using published data and estimated mechanical drag at higher and lower speeds, my testing today at 55 mph with a purely inline head/tail wind of 15 mph is just way off the mark even when accounting for the higher mechanical drag going with the wind vs. against it relative to actual air speeds (40 vs 70). Trying to pad them with an error correction or baseline consumption from zero is still rather specious in terms of where results should lie.
Clearly I have to resort to alternative means. Fortunately I do have one that is relatively easy with the range extending generator and an exceptionally accurate gas gauge for it's 2 gallon tank. I should be able to preserve a set state of charge with it and just run it down the same percentage every time or just run it dry. So I guess that means this EV driver will soon be talking MPG like everybody else for testing purposes anyway.
So far this thread seems more about getting good data than anything!