[Sven7]

Those things make frontal area skyrocket and allow large pockets of turbulence to form behind them. There's a reason people's front wheel skirts are fitted tightly to the car- if they stick out into the airflow they may negate aero gains or even hurt FE. The only reason to have monstrous flares, covered or not, is to run monstrously wide wheels legally (not protruding from the bodywork). Rally cars have to legally drive from stage to stage so they must comply with local laws. And of course, their main goals are handling and acceleration, not top speed or fuel economy.