Times are hard so we have to save money. If you have a car, one of the ways you save money is calculating your km/l performance, i.e. mileage, every time you fill up and find ways to increase it. Yes it helps a lot, but is it the be all and end all? Maybe there is a measurement thats slightly more accurate.
I though that well, since a lot of people are paid in monthly wages, maybe you should be looking at your transport related expenses also in a per month manner. You could measure liters per month or even better money spent on gas per month.
The old km/l is pretty good as it is but there are a times where it can be inaccurate. What if instead of your inner city commute with the traffic jams, you took a long cut that happens to bypass traffic because it uses a long stretch of highway? What if your usual routine involves a route that gets stuck in traffic (to work) and a less than 1km commute with no traffic (maybe to a mall or something) and you decide to walk that < 1 km? Both would negatively affect your km/l values but would decrease the money per month value.
Think about it.