Well, South Dakota I-90 where Tesla put their chargers is a single 75mph on a very busy two lane interstate with a lot of truck traffic.
Minnesota is a single 70mph on I-90. Thinking about it, I haven't seen a split speed limit in the upper plains.
"You in a Tesla driving 65 in the right lane is not going to affect traffic much."
"In the wide open spaces, there's enough room it doesn't matter."
You are going to be a nice blocker in traffic that generally flows along at 75 - 85 mph. It is a very busy truck corridor and you going 65mph is going to be a problem.
This is a good point, I wonder what the energy burden is going to be from that. You also need to factor in the impracticality of travel in the winter season (ie, near 0 deg F) in the east, or the very cold desert west of the Continental divide, particulalry at night (which is also extended in the winter).
It gets into the negatives for a lot of the winter. As I type this, it is -4F in Sioux Falls (just off I-90) with a mild 5-10mph wind with an expected low of -8F tonight.
There are a lot of windmills near I-90 since you can pretty much guarantee a constant wind.
I can assure you that doing 65mph on I-90 through South Dakota is a really, really bad idea. Its listed as 75mph and that would be the low-end for what people do. Nevermind the wind that is probably going to kill your range.