10%?!
I find that highly improbable.
The additional area of the mudflaps may produce extra drag but the magnitude of this relative to the overall drag on the car is likely to be quite small, especially given that they are placed after a large drag item that is the wheel anyway. I don't dispute that they will modify airflow but I believe any effect on efficiency will be minimal and quite possibly negligible in comparison to that from driving style, road and climate conditions, tyre pressure, etc.