Modern cars all still have catalytic converters. These converters require the fuel air mix be set on the rich side wasting some gas in the interest of cleaner emissions. The reality is though for quite some time computer controlled engines with fuel injection have been clean enough on their own that we do not need catalytic converters.
In addition these catalytic converters weigh alot. Vehicle weight is a primary determinant of fuel efficiency. To me it appears that liberals never want to see a government mandated program be terminated. It really appears as if they feel it would set a bad precedent to end a program. They apparently feel that if we manage to kill one government program that we the pleebles might get uppity and demand a second and a third erased. Quite likely true.
For this reason even when there are serial hybrid cars that get very high fuel efficiencies I wonder if the converters will go extinct. I suspect it will require quite an effort to kill them when it should be easy as can be.