My sympathies, to some extent lie with the manufacturers. Governments, generally not particularly gifted with intelligence, tend to enact laws, which are in conflict with the Natural Laws, on the basis that "You can always get round these, by research and design". As an example of this, I was once told that an American State (I've no idea whether this was true, or not), enacted a law declaring the the value of pi, would henceforth be 3, saving all those tedious calculations (Why they didn't pick 4, I've no idea, it would have been so much easier). The manufacturers were told, to make it like that. It's now the Law. The manufactures were also faced with other problems. More Laws stated that the cars must be more robust (heavier) to protect the occupants from the results of their actions, although not the unfortunate pedestrians/cyclists/etc they collided with. The buyers wanted more comfort (heavier), air conditioning, etc, etc.
In my younger days (so many days ago), as part of my Ordinary National Certificate in Mechanical Engineering, I did Heat Engines. Our lecturer, and he was a very good lecturer explained that the process of extracting mechanical power from combustion was largely a matter of creating a mass of hot gas and extracting the mecanical enrgy by cooling it down. The higher the temperature you get at the start and the lowest temperaturte at the end would determine the amount of energy you would get. Ther were natural Laws which determined how much of the available energy you could extract. The rest had to be discarded. And there was always a lot which had to be thrown away. Large installations, like power stations could theoretically use some of this for district heating and other uses, but since nobody wanted to be near a power staion, the resutlatnt heat in the medium of hot water, was usually too far from any possible use to be economic. The New York steam heating, so beloved of film makers was one of these, I believe.
The lecturer told us that there were research projects into ceramic engines, capable of the required qualities at high temperatures. Later I went to work for Ruston and Hornsby, who did just that. They did not have a ceramic engine, but developed one which ran at much higher temperatures, and therefore extracted more power . It worked fine with pure fuels (Distillate fuels), but when they extended it to the more common residual oils, it failed. The wear rate under the high temperatures and the heavy metal contaminents in the fuel, was horrendous, and the project failed, and with it the Company.
The auto manufacturers, presumably left to their own devices to compile the test schedules, did what so, so many organisations have done throughout the history. They wrote the schedules to comply with the letter of the Law. Look back to all those road tests and fuel consumption figures, going back donkey's years, in cars stripped of all removeable weight, on new engines very carefully run-in, on tyres pumped up to brick hardness, and in weather noted to be good for fuel eficiency (the colder, the better). A similar situation is coming to light as a result of the Grenfell fires.
As a result, as someone has pointed out. You get adherence to the law (just), but oh, where has all that power gone. I once owned a motorcyle, a real powerful beast. It was happiest at about 80mph. It would have been happier faster, but the effort of hanging on against the power of the wind sapped ones energy. It regularly turned out 50 miles per gallon. On one occasion, for an extended period, I had to follow another vehicle at about 40-45 mph, and was surprised to find I was getting better than 70 miles per gallon.
It used to be said that The Law is an Ass. Nowadays, it's more like The Law is ALWAYS an Ass.