Best efficiency speed can be understood by considering impact of fixed losses like HVAC, lights, computing, which consume more energy the longer you drive (so the total energy for those would be minimized by going faster) and the aerodynamic losses in which force goes with speed squared, power w/ speed cubed, energy over a fixed distance back to squared because at higher speeds time is reduced (this component is theoretically minimized by going very slowly). In between is rolling resistance which requires a fixed energy per distance, so it doesn't care about speed.
For EVs, the drivetrain efficiency is so high that it's variability with operating point doesn't affect this calculation much, and so the most efficient speed of an EV is around the speed at which the fixed losses equal the aero ones. This will vary greatly with environmental conditions since AC or heating load can be large in hot or cold conditions but at the right temperature will go to near zero.
In ICE cars, the drivetrain efficiency is much lower and so the drivetrain efficiencies are a much more significant part of the optimization problem, but the basic physics of the aerodynamics are the same.
The model I used to use in my head is that for an ICE, the most efficient operating point is probably around the lowest speed the car can operate in the highest gear, so maybe around 40 mph / 60 km/h? Obviously a rough heuristic though.
