For the 1984 Ford Probe IV concept car,a 9% drag reduction was claimed for the inner-fender/flexible urethane skirt combination used by Ford/Ghia.
This might be considered a drag reduction maximum for front wheel enclosure.Don't know.There's so little empirical data for such things.
It's a sure bet though,that the rectilinear flow on AeroCivic is going to produce about the best sink flow we could imagine.Not only is all the pick and shovel streamlining accomplished,basjoos has worked down into the macro-flow environment.
And it tells at the gas pump.
It's the gold standard so far as I've seen!