It seems that it is generally accepted that ethanol increases fuel consumption as its specific energy is lower than standard petrol.
When fuel consumption figures are quoted for cars under the new scheme recently introduced, what fuel are they allowed to use for the testing? If it were me, I would put "this vehicle is designed for 99RON fuel but can run on 95RON E10" in the handbook straight away as a "get out of jail free card".
If E10 reduces mileage by, say, 15% over pure super unleaded, will the extra fuel burned outweigh the claimed CO2 emission savings?