1. I was speaking of my full size front tire in an M5 boot (aka trunk) which by definition means it is NOT in a passenger compartment. If you are keeping a spare in the cabin (like the X5) then yes, you need to secure it. And really you should secure luggage, etc as a matter of course. Most dont.

2. A full size front tire/wheel will NOT fit in the E70 X5 spare tire compartment. The only thing that fits is the custom compact (skinny) spare. An 18", 19", 20", 21" rim ALL will **not** fit in this location. Has nothing to do with the rim size.

3. Buying a loose BMW 18 or 19" wheel is usually pretty cheap- personally I really like the quality that comes from OE rims.