Understanding fuel economy units: mpg, L/100km and km/L
Why the same car reads three different economy numbers, how the units relate, and how to compare a US mpg figure against a litres per 100 km rating.
Three units, one physical fact
A vehicle burns a fixed amount of fuel over a fixed distance, but that fact gets reported three different ways depending on where the car was sold. Miles per US gallon counts distance per unit of fuel, so higher is better. Litres per 100 kilometres counts fuel per fixed distance, so lower is better. Kilometres per litre also counts distance per fuel, so higher is better again. The calculator accepts all three and converts each into litres per kilometre behind the scenes.
Converting between the units
The relationships are fixed once you know that one mile is 1.60934 kilometres and one US gallon is 3.78541 litres. To turn mpg into litres per 100 km, divide 235.215 by the mpg figure, so 25 mpg is about 9.41 L/100km. To turn km per litre into litres per 100 km, divide 100 by the km/L figure. Because these are reciprocal relationships, a small change at high economy matters less than the same change at low economy.
Why per-100km readings feel counterintuitive
Drivers raised on mpg often find L/100km backwards, because a bigger number means a thirstier car. The upside is that fuel-per-distance scales linearly with cost, which makes budgeting simple. If your car uses 8 L/100km and fuel is 1.80 per litre, every 100 km costs 14.40 flat, exactly as the metric example in the tool shows. That direct link is why many fleet and tax systems prefer the per-distance form.
Getting an accurate real-world figure
Manufacturer economy ratings come from controlled test cycles and rarely match daily driving. For a truer number, fill the tank, reset the trip meter, drive normally, then refill and divide the distance by the litres or gallons added. Feed that measured figure into the calculator rather than the sticker value. City stops, roof boxes, cold weather and a heavy right foot all push consumption up, so a small safety margin on the estimate is wise.