`kg of fuel per metre` is division, not comparison. You can divide different units by each other. It isn't guaranteed to always make sense but it's very useful.
Hey, wait a minute-
The result of subtraction is a difference. In my mind this is the most basic way to compare things. Subtraction of differing units is illegal.
The result of division is a quotient (day to day we say ratio). Division of different units is legal but not always practical.
These aren't pure dimensionless numbers, they come with units of measure attached.
If there's need for good scalar try: https://en.wikipedia.org/wiki/Dimensional_analysis for https://www.sciencedirect.com/topics/chemistry/dimensionless...