Its not that simple. If you just consider the ENGINE in any car by
itself, it will be most efficient when its at its maximum power output.
But in a car, you don't need that maximum power output except during
all-out acceleration or driving at the absolute top speed of the car,
which never happens. If you put a teeny engine in a car so that its
maximum horsepower output was *just* enough to move the car at 70 mph
through the wind, then it would be super efficient, but couldn't
accelerate fast at all, couldn't go uphill at 70 mph, etc. So you need a
n engine with reserve power, which means that most of the time you're
running it at way, way less than its maximum theoretical efficiency.
When you do THAT, the best way to get economy out of it is to just turn
it as slowly as possible which is why overdrive is pretty much standard
on every car nowdays.

So the real answer is that the "particular speed" that a car is most
efficient at is sorta where the increasing efficeincy of the engine as
you demand more power from it crosses over the decreasing efficiency
caused by wind drag. Car builders put that point around the average
highway speeds that the cars will see for best overall results.