units - Why can fuel economy be measured in square meters?

With help from XKCD, which says

Miles are units of length, and gallons are volume — which is $\text{length}^3$. So $\text{gallons}/\text{mile}$ is $\frac{\text{length}^3}{\text{length}}$. That's just $\text{length}^2$.

I recently realised that the units of fuel efficiency are $\text{length}^{-2}$ (the reciprocal of which would be $\text{length}^{2}$) and I can't work out why this would be, because $\mathrm{m}^2$ is the unit of area, but fuel efficiency is completely different to this. The only reason I could think of for these units is just that they were meant to be used as a ratio; but then again, ratios are meant to be unitless (as far as I know, e.g: strain).

Please could someone explain why these units are used.

Answer

Imagine that you have a tube laid along some path and that the tube is completely filled with the fuel that you would spend to cover that path.

Area of the cross-section of that tube is the area you're asking about.

Now, if this area is bigger, the tube is thicker, which means more fuel. That is, more fuel to cover the same distance, which means lower efficiency.

Therefore, efficiency is proportional to the inverse of the area of that tube and that's why it can be measured in inverse square meters.