The same car gets a bigger number on two of these scales for being more efficient, and a smaller number on the third - genuinely confusing when you're comparing a US-spec car's MPG to a European car's L/100km rating.
MPG and km/L both increase for a more fuel-efficient car - more miles or kilometres squeezed out of the same fuel. L/100km moves the opposite direction on purpose: it measures how much fuel a fixed distance uses, so a more efficient car needs less of it, meaning the number goes down. A car that "only gets a 25" sounds worse than one that "gets a 35" - unless the 25 is L/100km and the 35 is MPG, in which case the 25 might be the more efficient car. Seeing all four side by side, with the direction stated plainly, is the only real fix.