It’s ALREADY here in the United States. The EPA (not the state legislature) has been dictating Texas highway speeds for over a decade. Highway 59 is limited to 55MPH in Houston (it was even 50MPH for a stretch for awhile) even though they’ve admitted that Houston’s air pollution problems are not due to or (resolved by controlling) traffic pollution. The bad things roll in (it is a flat landscape) and additionally there is the petrochemical industry (but it is not the sole source of the pollutants that enter Houston’s airspace).
I don’t know where “55 is most efficient” came from.
Several variables... RPM at final drive ratio... coefficient of drag.... etc.
Since drag is different for every vehicle, that number cannot be absolute.