When they are collectively forced to. As things stand, if manufacturers don't achieve an average of less than 130g/km for their entire production in the existing tests, they get fined. So obviously they optimise their cars for the tests. They'd be stupid not to. There is talk of a new regime of tests starting in 2017, because from April 2017 car annual tax will no longer be CO2 based. Meanwhile, the best measure and the best comparision is www.honerstjohn.co.uk/realmpg/ which, with contributions from 95,000 drivers, gives a Gaussian average of the economy that cars are actually delivering in real life.