When I upgraded the tires so that the spare didn't fit under the truck, (which is a lousy place if you actually need it, say, in mud, or on rocks, etc...), the first place I put the tire was on the roof.
I just slid the cross bar back to pin it, and ratchet strapped it down.
It held great, no movement, quite secure, etc.
It made the truck more tippy off camber and in emergency maneuvers, etc....too high a polar moment (Think of it as a weight at the end of a lever going from the tire on the roof, to the truck's roll center...with all that leverage to pull you over)
It was a PITA to actually get the tire down/back up there....you DON'T need a ladder, just open a passenger door, and step onto the threshold, and then onto a rear tire...and you can get at it.
My MPG plummeted, its another wall to push through the air...and it breaks up the laminar flow of air over the roof...too much wind drag.
I took it down, and put it into the cargo hold..but, I carry a lot of cargo, so that wasn't working either...but handling-wise, laying the tire flat, brake side up, to leave a large bowl facing up...worked best as far as COG went...and the bowl part was at least a place to stash stuff....
...if I had a little less cargo to load, I might have just left the tire flat in the cargo hold.
So, I ended up biting the bullet, and getting a rear bumper/tire carrier, which solved the problem.
So, of ALL the solutions, the roof for the spare is the worst....the money you save on the cheap mount is spent on gas anyway....its like RENTING a place for your spare.
:wink-big:
