This is not complete true. Most combustion cars perform best around 90km/h. Below that speed air resistance is not much of an issue but gears help a lot getting the best performance out of the engine.
For this reason the target speed for efficiency had to be 56 mph. For that would be important for comparisons. Fleet buyers were important in those days so a 'rep-mobile' able to sip the fuel as it went about crossing the country at sensible A road and motorway speeds was what was wanted. If a manufacturer wanted to sell into this market then the 56 mpg figure had to be available.
I am not sure fuel efficiency was discussed in quite the same way in the U.S. market during these times and there weren't the same tax incentives to get companies buying 'rep-mobiles' so the auto-sector could be stimulated.
Europe wasn't the EU in those days with each major country having different tax regimes for cars.