Driving an electric car for 4.5 years has helped me experience the tradeoffs in speed and efficiency (range) on a daily basis with fast feedback.

Though 55 mph is much more energy efficient than 65 mph, 35 mph is much more energy efficient than 55 mph as well.

I think the 55 mph point is realistically a balance between energy and time efficiency, not an optimum of energy efficiency alone.

Spark ignition ICEs experience something called “pumping losses”: There’s a physical impediment (the throttle) that the engine has to work against at low engine speeds. So while drag increases with speed, it’s also true that at lower speeds you’re still converting hydrocarbons to waste heat at the same rate. The truth is that fuel economy is a delicate balance between rolling resistance, aerodynamic drag, gearing, and the engine power curve.

With the car I usually drive the minimal efficient speed is around 30mph. Below that you are forced to a very low gear.

The tradeoff will be different for electric cars.

Get a car that shows you current fuel efficiency. You'll see that most efficient means as slow as you can suffer through.

“55mph is generally considered optimal for vehicle fuel consumption.”

For a while I did experiments with this and the most efficient speed was much slower. 55 is more about safety I would think or it’s possibly totally arbitrary.

Completely incorrect. Rolling resistance is largely independent of speed, and drag is quadratic in speed. 55mph represents a speed where drag dominates rolling resistance.