Fuel economy is quoted as miles per gallon in some countries and litres per 100 km in others, and the two run in opposite directions - a higher mpg is better, a lower L/100km is better. 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.
Divide 235.215 by the mpg figure for US gallons, or 282.481 for imperial gallons. Thirty US mpg is about 7.8 L/100km.
It depends on the unit. Higher mpg is better because it is distance per fuel. Lower L/100km is better because it is fuel per distance. They run in opposite directions.
Each of these has its own page with the exact factor, a reference table and the working shown. The converter above handles any of them too - these exist so the specific answer is one click rather than a search.