text: As for the valve stem cover. Yes, I've lost mine behind there, too. I had to back up so as to spin the wheels 180 degrees to get them out.
  The bad news is that I think yours is long gone. With enough starting and stopping, I'm willing to be that it fell out by now. Sorry.
  I put mudflaps (from coastal tech) on my 2004 Prius. They do stick out in the airstream but they look very small in relationship to the whole car. They do a good job of keeping stuff off the the areas of the car behind the wheels. This time of year it is mostly mud and manure. (I live in cow-country and all of the farmers manure does not make it to the fields, some of it ends up in the road.)
  About how much extra gas would you suppose that I will burn over the life of the car? I do very little high speed driving, usually in the 40 to 60 mph range.
  Have you seen those disk hubcaps on the Prius that Toyota shows in the ads. I am refering to the Prius that goes very, very fast.
  You know, I'm sure the mudflaps will not affect your yearly milage all that much but observing wind tunnel aerodynamic tests for years I know that with the installation, the whole unrestricted flow would be destroyed. Ditto with a bike rack mounted anywhere on the exterior. The rear mounting probably would completely destroy the vacumn behind the car created by the spoiler. Milage though would be hard to measure. Not enough to worry about.Someone on this net may be able to again post the URL that was here last year of a wheel company that when selected will show you a dozen different wheel selections actually on a Prius at stop and in rotation. I've lent all my research papers to a friend researching the Prius so I don't have it now.
  Thanks to all, especially Hyperion and Bobc, for the info. I'm glad to see that I'm not the only one to lose a valve stem cover (stole one off a bike and we're back in business).