In all seriousness, it should be based on what is safe, not what is efficient. You can chose an inefficient car, why not inefficient speed? Besides the speed limit would be more like 60Km/hr if it was all about absolute saftey and efficiency. I bet it has been 100 for generations going back to when cars and roads were no where near as developed as they are today.
https://www.google.com/url?sa=t&sour...=1557577988188