... Do you recommend a certain highway speed sweet spot? ...
... Can you get the car to run in EV over 70?
It is pretty much a proven fact that just about any car will get better MPGs at 55 MPH than it will at 65 MPH; better at 65 MPH than at 75 MPH, and so on and so on. It is basically just a matter of the physics involved which says that to keep a mass moving at a higher rate of speed requires more energy, and since it is not a linear increase in the required amount of energy for a given increase in speed, there is a reduced ROI for driving at higher rates of speed.
My experience is that the slower that you drive an FFH, the better the MPGs will be. From my empirical experience, I seem to get better MPGs at 35 MPH than I do at 45 MPH. And I also seem to get good MPGs at 25 MPH, but I have not driven nearly enough miles at 25 MPH to know whether I am getting better MPGs than I do at 35 MPH, since it is only 2/10 mile from the last major street to my home, which is 25 MPH zone.
So since 35 MPH is about the slowest I can drive and still get around in a decent amount of time, and without upsetting the other drivers around me even more than I already do, 35 MPH seems to be the sweet spot for me and my FFH, and in my particular driving conditions/environment.
BTW, I don't ever plan on driving over 70 MPH in my FFH, ever, even if the speed limit is 75, 80, or even 85, since I believe 70 MPH is plenty fast for any conditions, and much safer than any speeds faster than that. And at speed limits which are lower than 70, I rarely drive my FFH more then 3 MPH over the limit, and usually right at or below the speed limit these days.