Fuel consumption

In the UK, we traditionally measure car fuel consumption in miles per gallon.  Actually, this is a measure of fuel economy - higher numbers mean greater economy and less fuel consumption.  The Continental convention of litres per 100 km is a measure of fuel consumption.  It is interesting to look at the dimensions of this quantity.  Litres measure cubic capacity, so they have the dimensions of length cubed, while 100 km has the dimension of length.  Fuel consumption therefore has the dimensions of (length cubed / length) which is length squared, or area.  It is thus possible to measure fuel consumption using units of area, such as square millimetres.  For example, a fuel consumption of 7 litres/100 km (about 40mpg)  could be written as (7 x 106) / (100 x 106) = 0.07 square millimetres.  This does have a physical interpretation: it is the cross-sectional area of a trough running along the roadside from which the car could scoop up fuel as it went along.