Like you, we've been noticing a massive range drop in our 2016 90D Model X. We did some manual calculations.
To drive 14 miles this weekend, our X dropped in range from 125 miles to 104... (A range drop of 21 miles to only go 14 miles.) Not on freeways, flat FL roads, no crazy driving, not towing, they recently did an alignment, nothing attached to the outside of the X, etc. We had been running calculations on it. It's taking that percentage consistently. (My husband actually guessed how much we would have when we got home from the charger, and he nailed it... That's how consistent the #s are...)
Based on the charge we left our home with, our weekend outing on Sunday should have had us stopping one time to recharge to make it back home. We had to stop a total of 4 times. The last leg was the one we ran the test on....
Now, the question is HOW do we get Tesla to agree that the HV has issues. And how the heck do you contact them about warrantee issues....