They conduct their own tests to guidelines. See: www.honestjohn.co.uk/realmpg/. The ECDC figures are the only figures that manufacturers are legally allowed to publicise. So, as a service, we set up Realmpg more than two years ago, where real drivers could record what they are really getting for the benefit of everyone else. Another fact is that if manufacturers do not achieve a corporate average of less than 130g/km CO2 in the ECDC tests, they are now fined. This explains why manufacturers have concentrated on the ECDC tests at the expense of fuel economy in real life. And why many larger petrol-engined models have now gone missing from manufacturer's ranges.