I believe that changed a while ago albeit participation by the manufacturers was on a voluntary basis. In ye olde days the measurement was taken at a permanent 56mph on a rolling road and, surprise surprise, vehicles were tuned to be very fuel efficient at, you guessed it…56mph.
The current standard is real world, fixed route driving, although I’m not sure if it is undertaken independently.
Amazing what a difference a heavy right foot can make…I had a Civic Type R for a while when they first came out…..lucky to get 20mpg if pushed hard, could get over 50mpg with care. It was a company car……