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I don't doubt your experience, having been frustrated many times myself, but in case anyone is curious, my first and most obvious guess worked:

It’s per 100km. I tried a few prompts, wolfram didn’t get it right!

Sorry! Totally missed that! Indeed it seems more difficult.

Because way down in the "Corresponding quantities" it shows "0.47 L/km" and "47 L/100 km"

:)