Don't know what rack or basket you have for your roof, but I have the Yakima Load Warrior and I use Yakima's spare tire carrier ( http://www.realcyclist.com/yakima-sp...8-001b2166becc ) to secure the tire to the basket. Once that thing is sinched down that tire isn't going anywhere, plus it has a place for a lock to keep it from wandering off.
"The major difference between a thing that might go wrong and a thing that cannot possibly go wrong is that when a thing that cannot possibly go wrong goes wrong, it usually turns out to be impossible to get at or repair."
-Douglas Adams, Mostly Harmless