system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: I've heard European cars are more advanced and fuel efficient than American cars. Why are these cars not sold in America?

It's obvious that fuel efficient cars exist, why aren't cars sold in Europe also sold in the US? And I don't mean brand names, I mean specific models of car. 

Can one buy cars and Europe and sell them in the US? How would that work?
gpt
Emissions regulations are different and the test cycles used to determine compliance are different. Therefore, it takes more effort to design and certify for both Europe and the US.

Safety regulations are different and the crash tests used to determine compliance are different.  Therefore, it takes more effort to design and certify for both Europe and the US.

Roads are older and narrower in most of Europe.  Europeans favor smaller cars which make is easier to get around.  Americans favor larger cars for comfort, style, and to haul their things.  Therefore, demand is lower for smaller cars in the US, making it hard to justify the costs to import.

Fuel is more expensive in Europe.  Again, this means that Europeans will favor smaller cars.  Gas is relatively cheap in the US, reducing demand on small cars and making it hard to justify the costs.

That being said, car companies are moving more towards "world" cars to reduce costs.  One example is the Chevy Cruze that is sold nearly identically throughout the world.