"20 MPG is about 0.1 square millimeters," according to https://what-if.xkcd.com/11/. "If you took all the gas you burned on a trip and stretched it out into a thin tube along your route, 0.1 square millimeters would be the cross-sectional area of that tube."

It's simple dimensional analysis (https://xkcd.com/687/).

So, X litres per km is X * 10^-3 m^3 per 10^ 3 meter. Which is X * 10^-6 square meter.

Cool. So my car has a fuel efficiency of 0.000007 square meter :D I love it.