I do agree though, it is not easy to compare especially as the tests used by the manufacturers to provide the figures will also differ between countries.
What would make things a LOT easier for us Brits to understand fuel efficiency would be to quote petrol cost in £s per gallon, efficiency in litres per mile, or fully switch to km and metric (I'd prefer the latter).
While our roads are denoted in miles, our car magazines quote mpg but our petrol pumps sell by the litre - it's a mess that means we're never clear exactly how far we'll be able to travel for our £.