Automobile fuel efficiency is increasing. How high can it go?

In the US, automobile fuel efficiency standards have been increasing - and will go to 40 miles-per-gallon in 2026. Clearly, automobile fuel efficiency is increasing to make this possible.
  •   Is electrical technology driving this improvement?
  • How high can automobile fuel efficiency go? 
  • What technological breakthrough will be required? 
  ·       EPA's New Emission Standards to Mean 40-MPG Fleet Average in 2026 (caranddriver.com)