Fuel economy is a function of weight (size) and efficiency. Todays technology is pretty close to the maximum efficiency for a Rankine Cycle (Internal combustion) engine. Without a new engine, the entire car will have to be made of rice paper and only allow one passenger.
I believe the OBAMA CAFE standard for cars and light trucks in 2025 is 54.5 mpg. There are almost NO vehicles today that can meet that standard. I think the Prius does but if you are over 5”5” tall, forget it.
The Volkswagen diesels are very close to 54.5 MPG but to meet the “average” it’s going to have to be close to 100 MPG to balance with the pickups.
As much as I hate the Prius, Ford stuck me in one while they waited for parts for my Windstar during the recall. It was slow as snot but my 6 foot body fit in it just fine.