According to XKCD, Gas mileage can be measured in square meters: Miles are units of length, and gallons are volume—which is length^3. So gallons/mile is length^3/length. That’s just length^2. [thus] Gas mileage is measured in square meters. Source: Droppings Love the conclusion at the end as well.
Not quite. If we could collect all the poop from all the birds in the world, and use it in our cars, at 13 MPG there would be no bird poop left to stain the panoramic roof. At 15 MPG there'd be stained roofs. Conservation of bird poop is bad!