The mathematical units for fuel consumption of a vehicle (MPG, L/100km) can be rearranged and/or simplified down to be expressed as an area. This area corresponds to the area of a "string" or "tube" of fuel, on the path of travel, that would be needed for the vehicle as it drives along it.

Source: https://www.youtube.com/watch?v=kkfIXUjkYqE (0:35)