Some of you have pulled the spare off the truck and put in a propane tank in its place. Did you mount the spare elsewhere on the truck? I've been thinking about this......what about a swing-around mount for the spare on the rear? Has this ever been done before? What about mounting it on a roof-rack? (on a crew cab). Just brainstorming over here.....:)

Depending on how you mount you tire on a roof rack you will have a lot of wind drag. Not to mention the weight of the tire to have to take it off or put up there.
I would say the bed. It is easy enough to take out if you want it some where else or need it.