Using liters/100 km, it's a tad easier, we just have to get the orders of magnitude right - liter = (100 mm)^3 = 1e6 mm^3 - 100 km = 1e8 mm So, 10L/100 km is

Using liters/100 km, it's a tad easier, we just have to get the orders of magnitude right - liter = (100 mm)^3 = 1e6 mm^3 - 100 km = 1e8 mm So, 10L/100 km is 0.1 mm^2. (Note that we want low values, and this is on the high side for European cars - the usual advertised numbers for sedans are half of that, with reality somewhere in between.)