I have a P85D and have a question about range. If you are trying to squeeze the absolutely longest range out of the car what is the optimum Interstate speed to drive.
Look at JB's post Driving Range for the Model S Family | Tesla Motors and the related, earlier post Model S Efficiency and Range | Tesla Motors - so somewhere around 22mph seems to get you the best range - yet I hope you are not planning to drive 22mph on an interstate... actually, even going 40mph is rather dangerous
Bear in mind that steady power is more important than steady speed. It's okay to lose a bit of speed going uphill and gain a bit going downhill.
This has been a huge topic on these pages. Search for the 2015 record setting trip across the country. Sorry can't remember the exact name and I if leave this page, I'll never find you again. There are lots of super smart guys determining what the best speed to charging ratios are. I'll look for the tread and send it to you.
For quick reference I am finding that in moderate weather on flat ground with no wind I can get the "Rated Range" at 65 MPH.
I don't think the OP is trying to get between Superchargers. I read it as traveling on an Interstate without Superchargers, how fast should they drive to get the farthest on a charge. In Texas, not all of our Interstates and sideroads and driveways and dirt paths are equipped with Superchargers like they are there in CA. But we're a patient bunch.