The manner of test for official economy rating is laid down and all manufacturers abide by it. The fact that the tests do not emulate the real world is not the fault of the manufacturers. Just take the official figures as a guide to relative economy of different vehicles. As for emissions the same applies. Whether that should result in higher band Road Tax is another matter. Even the much quoted 'Toyota Prius' would not make it's current tax band if 'real world' figures were used.