It is so sad when an icon is found to be wanting.
But I can confirm your observation that MMC has overstated their data. This has serious financial consequences in the UK where annual car tax is based on CO2 emissions. I pay £505 per year based on the CO2 output of 282g/km.
Based on your evidence, which I can largely confirm, the figures are overstated by 15%. Reducing the CO2 emissions by 15% would cut my annual tax by £!5.00!!!!
Another good reason to run older cars, at c.34mpg / 200g/km my 4/4 should pay VED of £290, but because it is pre 2001 I get away with paying only £230
