Three ways to express the same underlying efficiency
Fuel economy is simply distance traveled divided by fuel consumed, but that ratio gets expressed three different ways depending on the region and convention: kilometers per liter (km/L, common in India), miles per gallon (MPG, common in the US), and liters per 100 kilometers (L/100km, common in Europe). All three describe the identical vehicle performance — they just divide the same two numbers in different orders and units.
Why higher is better for two of them, but lower is better for the third
km/L and MPG both measure how far you get per unit of fuel — a higher number means better efficiency. L/100km flips the ratio around: it measures how much fuel you burn per fixed distance, so a lower number means better efficiency. Mixing these up is an easy mistake — a "smaller" L/100km figure is actually the more efficient vehicle, the opposite of how km/L and MPG work.
A worked example converting between all three
A vehicle that covers 500 km on 40 liters of fuel works out to 12.5 km/L, 29.4 MPG, and 8 L/100km — three different-looking numbers describing the exact same fuel efficiency. Notice that 8 L/100km is a smaller number than 12.5 km/L despite describing a vehicle that's easy to mentally compare as "decent mileage" either way, which is exactly the kind of mixup that makes cross-unit comparison error-prone without converting first.
A second example starting from miles and gallons
A vehicle that covers 300 miles on 10 gallons works out to 30 MPG, which converts to 12.75 km/L and 7.84 L/100km. This is slightly better fuel economy than the first example's vehicle (12.75 km/L versus 12.5 km/L, or equivalently 7.84 L/100km versus 8 L/100km) — the same small improvement shows up as "higher" in the km/L and MPG figures but "lower" in the L/100km figure, precisely because of the inverted direction.
Why this matters when comparing vehicles or trusting advertised figures
Advertised fuel economy figures are usually measured under standardized test conditions and can differ from real-world driving, but even before accounting for that gap, comparing two vehicles' fuel economy correctly requires making sure both figures are in the same unit — or converting one to match the other — since a direct number-to-number comparison across km/L and L/100km will give you exactly the wrong answer about which vehicle is more efficient.