The direction is the important part. At 6.0 L/100 km, divide 100 by 6.0 to get 16.67 km/L. At 8.0 L/100 km, divide 100 by 8.0 to get 12.5 km/L. The smaller consumption number is therefore the better result. Comparing 20 MPG with 30 MPG follows the opposite visual rule because MPG says how far one gallon carries the vehicle.
Worked example with numbers
Choose the unit used by the record you are reading. A US fuel-economy label commonly uses MPG; a fuel receipt in litres and a road distance in kilometres naturally support L/100 km or km/L. For a long trip, L/100 km often makes fuel quantity easy to estimate: 450 km at 6.8 L/100 km needs 30.6 L. For a fill-up log, km/L may feel more direct: 30.6 L over 450 km is 14.71 km/L.
How to make the calculation
Reciprocal units do not change by a fixed addition. Improving from 10 to 12 km/L saves 16.7% of fuel per kilometre, whereas moving from 10 to 12 L/100 km increases consumption by 20%. Use the MPG and L/100 km converter for a declared MPG reading, then state whether that MPG is US or Imperial before comparing it with a foreign specification.
Check the units and assumptions
A useful check is dimensional. “Litres per 100 kilometres” multiplied by kilometres leaves litres. “Kilometres per litre” multiplied by litres leaves kilometres. If a calculation gives litres squared or kilometres per 100 litres, the ratio was inverted. Round only after the conversion; rounding 6.35 L/100 km to 6.4 before a long-distance calculation changes the predicted litres.
Common mistakes
Do not say that 8 L/100 km is better than 6 L/100 km because eight is larger. Do not use 235.214583 for Imperial MPG: that constant belongs to US MPG. Avoid comparing one displayed decimal from an on-board computer with a carefully measured multi-fill average without naming their different measurement methods.
When this result does not apply
Fuel-economy measures describe energy consumed per travel distance, not total transport cost. They omit fuel price, tolls, charging, vehicle purchase, maintenance, and occupancy. A conversion is exact only to the precision of the input, and a real-world average changes with route, weather, speed, load, and driving style.
Source for the external fact
The U.S. Department of Energy’s fuel-economy label guide explains MPG as miles travelled per gallon and distinguishes annual fuel-use information from the efficiency figure. Its definitions help prevent a distance measure being read as a cost measure.
Related calculators
For a money scenario, enter the consumption value in the fuel-cost calculator; for a recorded tank, start with the real MPG calculation.