ORIGINAL: allanflowers
Some rear spoilers on cars do help in terms of lowering the CD. Probably all of them help in terms of creating down force for handling. Whether they are worth the money they cost and the weight penalty (which certainly doesn't help mileage) would depend on the vehicle. Porche 911s are terrible without the spoilers. Their rear surfaces droop off too fast and the air separates way up without them. The Audi TT was dangerous on the Autobahn before they fixed it with a rear spoiler. The VW beetle could be greatly improved with a proper rear spoiler (not the silly little automatic lip at the top of the backwindow on their "sport" model).
Does that stuff have any real effect on handling at the around-town speeds we are *supposed* to be going? I mean, autobahn is one thing - but you shouldn't be doing too much "handliing" there. I just think it's almost all about marketing - since a car's life is almost all spent at speeds where these things don't do much.