The speed difference is just too much on german Autobahn for instance. Also trucks will pass you and they are usually not happy if they are stuck behind someone that is even slower than them. So 70 is a nogo in most european countries on motorways.
Maybe we should offer a setting to define the lowest speed people want to travel on the motorway. For example on german Autobahn it doesn't make much sense to travel below 100km/h, but it is possible to go down to as low as 90-95 if it is really necessary to reach the only available charger there is for the next 300km. So maybe we should propose a speed limit minimum which is max 20% below max planned speed.
So 20% below max speed would then allow following min speeds:
100 -> 80
110 -> 88
120 -> 96
130 -> 104
I guess we could also round down (or up) to the next
value % 5 == 0 which would then allow following min speeds:
100 -> 80
110 -> 85 / 90
120 -> 95 / 100
130 -> 100 / 105
140 -> 110
150 -> 120
160 -> 125 / 130
Those sound like reasonable min speeds people would be willing to drive in exchange for reaching a better charger. I would prefer the round down, as most people tend to select a higher speed anyways and we still have a reasonable range of speed to expand the range a bit if necessary.