One kilometre per litre is 2.3521458 US miles per gallon. Both are efficiency units — distance per fuel — so unlike most conversions in this category this one is a straight multiplication.
About the units
Kilometres per litre is the metric efficiency convention, standard in Japan, India and much of Asia, Africa and Latin America. Miles per gallon is the American equivalent. Because both put distance on top and fuel underneath, converting between them means converting the distance and the volume separately and combining — a mile is 1.609344 km and a US gallon is 3.785411784 litres, so the factor is 3.785411784 ÷ 1.609344. No reciprocal is involved, which makes this the easier half of the fuel category.
The exact factor
The exact value is 2.35214583333 mpg per km/l. So 15 km/l is 35.3 mpg, and 40 mpg is 17.0 km/l. Note that the number 235.215 that appears on the l/100 km pages is this same constant multiplied by a hundred — the two are the same conversion viewed from the consumption and efficiency sides. Recognising that saves memorising two constants.
Where you meet this conversion
Comparing vehicle data between Asian and American markets, which matters for imported and grey-market vehicles. Japanese domestic market specifications use km/l throughout, and a car imported into the United States has to have its economy restated. Motorcycle economy is quoted in km/l across much of the world and in mpg in America. Shipping and generator fuel consumption in developing markets is often tracked in km/l or litres per hour, and converting to American conventions runs through this factor.