Fuel economy converter: mpg, l/100 km, km/l

By dividing 235.21 by the number of miles per gallon, if it is the US gallon. Thirty mpg is therefore 7.84 l/100 km, and forty mpg 5.88. For the British imperial gallon, a fifth larger, the constant becomes 282.48: thirty imperial mpg is 9.42 l/100 km. A figure that does not say which gallon leaves twenty per cent of uncertainty.

Because the average of an inverted scale is not the average of the numbers. A trip at 5 l/100 km followed by an equal trip at 10 l/100 km does average 7.5 l/100 km, since consumption adds up per kilometer. But in miles per gallon the same trips, 47 mpg then 23.5, do not average 35.3 mpg: they average 31.4. That is why American agencies also publish a figure in gallons per 100 miles, which does average.

From 15 to 20, by a wide margin, although the second jump looks twice as large. Over 100 miles, fifteen mpg burns 6.67 gallons and twenty burns 5, a saving of 1.67 gallons. Thirty mpg burns 3.33 and forty burns 2.5, a saving of 0.83, half as much. An inverted scale flattens differences at the top, which is why replacing the thirstiest vehicle in a household pays more than improving the most frugal one.

In kilowatt-hours per hundred kilometers, the direct analogue of liters per hundred kilometers: same scale, same direction, lower is better. A small electric car uses around 15 kWh/100 km, a large saloon 20 to 25. The United States prefers MPGe, a mile per gallon equivalent, by declaring that a gallon of petrol is worth 33.7 kilowatt-hours: that is an energy equivalence and not a measure of cost, since a kilowatt-hour and a liter are not bought at the same price.

In Japan, India, Brazil and part of South-East Asia, where the scale is the English-speaking one, distance per volume, but with metric units. Twenty kilometers per liter is 5 l/100 km, fifteen is 6.67 and ten is 10. The conversion is the same division as for miles per gallon, with 100 as the constant instead of 235.21.

The question has no single answer because it leaves out two numbers: the size of the tank and the consumption. Range is their product. A 50 liter tank at 6 l/100 km gives 833 kilometers, that is 518 miles. A 13 gallon tank at 30 mpg gives 390 miles, that is 628 kilometers. It is that same product your dashboard shows as range, worked out from the last few miles of driving, which is why the figure moves as you go.

You have to say which gallon first, or the number means nothing: the same car reads 30 mpg in the United States and 36 in the United Kingdom, the imperial gallon being a fifth larger. In US gallons, a recent compact sits at 35 to 40 mpg, that is 6.72 to 5.88 l/100 km; a hybrid goes past 50 mpg, under 4.70; a large SUV falls between 20 and 25 mpg, 11.76 to 9.41. A ten mpg gap is not worth the same at both ends of the scale, which the next question works out.