I disagree. As you pointed out, fuel economy is worsened when more fuel is being consumed per amount travelled. But the worse possible economy you can have with distance per fuel quantity is 0 [e.g. fuel being consumed but no motion], with higher numbers indicating improving economy. What is the worse economy you can have with fuel quantity per distance on a number scale? In addition, to measure ever improving economy you need more and more fractions between 1 and 0 rather than merely using higher and higher numbers.