This depends. The drag coming off objects such as wheels or struts is usually pressure drag (the drag associated with bluff-bodies or seperated flow). The drag coming off a well-streamlined shape such as a properly designed wheel pant is usually skin friction drag. Depending on the type of flow, Reynolds number, etc., the skin friction drag can often be much smaller (sometimes an order of magnitude or more) than the pressure drag without the fairing. Assuming again that the pant/fairing is designed properly with little to no flow seperation, the savings in drag can be very significant despite a doubling or more of surface area.