Always carry mine inside the car. I use a bit of cardboard from an old bike box to keep the carpet clean. I only need to remove the front wheel and it fits easily. Keeps it out of sight of prying eyes, doesn't get covered in road salt and grime delivered by 50-70mph road blast on your way, and doesn't affect your economy. However, I can only fit one bike in my car and means the loss of use of my rear seats. Some of my mates use roof bars and also highly recommend Thule. I don't do much mountain biking these days so it is mainly my time trial bike that I use now, which I wouldn't dream of carrying externally anyway plus it doesn't get the inside of the car mucky.
F31 330d M Sport
Previous Cars: E36 318is Coupe, E46 323ci, E46 M3, E92 325d M Sport