In case anyone is still working in mpg instead of Litres/100km, there really are good reasons to change.
One important reason is that mpg makes it hard to work out the savings that have been achieved with improvements in fuel economy.
Who has saved the most money?
Driver A improves her fuel consumption from 40 mpg to 60 mpg. (50% better)
Driver B improves her fuel consumption from 20 mpg to 25 mpg. (25% better)
In fact driver B saves more money than driver A. For every 100km they travel, driver A saves 2.4 litres while driver B saves 2.8 litres over the same distance.
Another example:
Both drivers improve their fuel economy by 5 mpg (from 40 to 45 and from 20 to 25). Driver B is saving 2.8 litres for every 100 km; driver A is saving just .8 litres over the same distance.
Another reason is that it is a bit illogical to talk about fuel consumption in mpg. Almost everyone finds themselves most of the time in the position of having a set distance they need to cover rather than having a set amount of fuel to work with. That is you need to know how much fuel you will use to cover that distance. Whether that is the distance you expect to cover to get from point A to point B, the distance you expect to do in a year or the distance you need to cover on that trip around Oz. If you know how many litres you consume for every 100km, it is dead easy to work out how much fuel you need.
It is fairly rare to find yourself in the position of having a set amount of fuel and you just drive as far as you can on that fuel, but that is what mpg lends itself to.
The same problems, distortions, lack of logic, difficulties and inconvenience apply to km/litre.
Litres/100 km really is the only sensible way to work out fuel consumption.

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1973 Series III LWB 1983 - 2006
1998 300 Tdi Defender Trayback 2006 -
- often fitted with a Trayon slide-on camper.