In most of Europe it’s not km/l but instead l/100km (number of liters required to drive 100km). It’s IMO a better way to frame consumption of resources.
I think we could reduce the litres / km (volume / distance) units to just square meters (area).
For example, 7 litres / 100km = 0.07 mm^2
It's the cross-section of a noodle of fuel laid out along the road that the car needs without a gas tank.
"How thick is your Spaghetti?" I really like that idea!