Ford has made a 65 mpg Fiesta ECOnetic. You won't find it in the US though.
The ECOnetic runs on diesel, which we tax significantly higher than other fuels. Despite higher efficiency, diesel has traditionally been reserved for more commercial drivers, due to its generally higher noise and odors. Taxes on diesel were originally designed on the assumption that we could target businesses with a road tax while giving consumers a break.
Unintended consequences in everything.
As advancements have made diesel cleaner and more appropriate for consumers*, the taxes now prevent the U.S. drivers from making the cleaner, more efficient, and lower emission choice.
*Industry shill page, read with caution. I link because I found the overview of technologies illustrative.