1. By their nature, ICE engines are only about 20-30% efficient. Meaning 70% of the gasoline that is burnt by them is lost as heat. Formula 1 ICE engines are about 50% efficient, but these engines literally cost tens of millions of dollars and need a specialized team of engineers just to start the motor.

2. Cars have gotten heavier, the engines are producing more power, and despite this they still get better fuel economy than they used to. So they have gotten more efficient when you consider all the variables.

The overwhelming issue as to why we can’t have a 100 MPG 3500 pound car is simply that by design ICE engines aren’t that efficient and we’ve almost reached the practical limit of what we can do with them.

We’d have to switch to a different kind of engine/fuel like nuclear powered cars.