Mega Fuel Economy Converter

Fuel economy is measured in two opposite ways, and this is the key to converting it correctly. The first is efficiency — distance travelled per unit of fuel, where a bigger number is better: miles per gallon (mpg) in the US and UK, and kilometres per litre (km/L) across much of Asia. The second is consumption — fuel used per unit of distance, where a smaller number is better: litres per 100 kilometres (L/100km), the standard across Europe. Because one is the inverse of the other, you cannot simply multiply to convert between them — you have to flip the value. That is why the well-known shortcut for US mpg is a division: L/100km = 235.215 ÷ mpg, and mpg = 235.215 ÷ L/100km. A second trap is the gallon: the US gallon (3.785 litres) is about 20% smaller than the UK, or imperial, gallon (4.546 litres), so the same car reads roughly 1.20 times more miles per gallon on the UK scale than on the US one — 30 US mpg is about 36 UK mpg. This converter keeps the two gallons separate and handles both efficiency and consumption units in one place. Behind the scenes every value is converted through a single common measure — litres per 100 km — with efficiency units inverted automatically, so any unit converts accurately to any other, whichever family it belongs to.