What about something that just covers the gears and derailleurs? If you cover the whole bike, it might act like a sail and not only slow you down, but use far more fuel because of the drag. Pygmy makes covers in lots of sizes. http://www.adventurecycling.org/stor...pygmy-pack.cfm
It won't protect the paint as much, but will protect the gearing. We've traveled all over the country with our bikes on the back of our car, and I don't know that I've really had any cosmetic damage to the bikes from the road. The biggest issue is keeping the chain/gears clean.