So, not actually a practical suggestion for the vast majority of people going on trips outside of town.
For sure, it's true that you will get much better energy utilisation at 85kph vs the speed limit of 100 or 110kph. One of the best things to do if you are in danger of not making it to the next recharging station is to SLOW DOWN.
But... that's a very impractical methodology to PLAN on going slower than the speed limit and peeing off the rest of the vehicles driving it, particularly the single lane highways where commercial vehicles are using it along with the rest of us.
It takes a deal of effort for a B-Double to overtake someone pottering along well under the speed limit and causes everyone else stuck behind them to get cranky and think about performing dangerous overtaking manoeuvres.
While you may be maximising your energy efficiency you're not being the most thoughtful person to the rest of the road users.
Anyway, that's my take on it and I apologise if you feel I'm being rude, but I think it needs to be said.