Relatedly, fuel consumption can be measured in square meters:

https://physics.stackexchange.com/questions/325733/why-can-f...

Yes, and the grandparent's calculation can be accomplished by simply dividing the fuel volume by the "area" of the car's efficiency (or multiplying if using MPG, which is inverse area)

https://www.wolframalpha.com/input/?i=%283m%29%5E3*24.9mpg

Pico hectares are a good unit of area to use for this. For a car that gets N miles/gallon, for 1/N gallons/mile, that works out to 235/N pico hectares.

That results in nice small, but not too small, numbers for most current cars, around 50 pico hectares for a Humvee in the city to around 4 pico hectares for a Prius on the highway.

Square centimeters or millimeters seem easier to visualise. The area represents the size of the tube that feeds the engine to get this mileage. You could even express it in the form of a radius.

I cannot really find a standard definition of pico hectares, but am I correct in assuming we're talking about 0.01 mm²? Or 1 square hecto-micrometre, I guess.

2 other numbers I read today on HN that made me look at things differently, although I knew them in other units but those didn't 'click' as much: air (including the water in it) weighs about 4 kg/m3 (much more than I would have intuited), and the whole US uses 'only' 1.3 km3 of water per day.