One year ago I part-exchanged a 2006 Citroen C3 for a new Citroen C3 1.6 HDi (April 2009) with the same engine. The previous 1.6 HDi engine gave me very good fuel economy, 60mpg with mixed driving and 70mpg on motorway driving at 65mph. The new C3 is less fuel efficient, returning 55mpg with mixed driving and barely 50mpg on motorways. My main dealer has checked the ECU and could not find a fault. The technician said that there isn’t a particulate filter with this engine, which was his original explanation for reduced fuel efficiency. He thinks the most likely reason is that the updated software in the ECU is the cause, to reduce the CO2 admissions. However the old engine produced 118 g/km and the new engine is 115 g/km, and I find it hard to believe this has made such difference to the efficiency. Have you any other possible explanation?