What's The Most Fuel Efficient Car Ever Made?

Since the dawn of automotive time, engineers have tried to make cars perform to their highest ability. That includes fuel efficiency. What's the most fuel efficient car ever made.

We're leaving the requirements fairly open here for most fuel efficient ever. Steamliners are open. One offs are open. Concepts are fine. The only requirement is that it's powered by a combustible fuel, so no electric cars allowed. And don't use a loophole that gas was used to build a Tesla. We're not falling for that one.

So, have at it: What's the most fuel efficient car ever made?