90s cars are just as efficient as new ones (neglecting hybrids). They have port-injected engines that run hot, lean, and efficient. They don't suffer from carbon buildup. They don't make nanoparticulate soot. But they do make more NOx, and NOx is what emissions regulations measure.

90s cars are also lighter, which again contributes to efficiency. Since then, cars have become heavier and gotten bigger blindspots in response to crash testing and the existence of SUVs.

In fact, merely talking about fuel economy, in a society that now mostly drives SUVs and oversized pickups, has become an absurd thing. We end up fretting about marginal issues while the primary trends thunder towards oblivion.