I've read that some people have bolted sheets of aluminum or corrugated plastic to the undersides of their cars to improve the aerodynamics. In general, what effect does that have apart from increasing FE? Does the plastic sheet accumulate oil and other fluids which can no longer drip on the street or driveway? Also, the sheet probably needs to be removed to access certain parts of the car; how much of a problem is this? Would it cause maintenance problems if car designers streamlined the undersides of cars?