Originally Posted by cssnms
After looking at those pics again, I wonder if I could mount that smaller bmw spare tire up under the car where the mufflers use to be. Hmmmm....
Yeah, that's what I was thinking too. You'd need to retrofit some kind of spare tire "lowering" linkage. My wife's 2005 Chrysler Pacifica has its spare/doughnut tire mounted this way. To lower it, you have to crank a bolt from inside the trunk to lower a steel cable holding the spare. This might be difficult to retrofit as you have the passive DEF tank (I think) mounted under the trunk (and above the stock exhaust).
The first thing to check is whether you have enough room for a spare. You'd need at least 25" in diameter for a 225/45-17 tire. Plus, if you don't have enough distance from the exhaust pipes, you could risk melting your tire. Maybe some extra exhaust shield
Here's some similar DIY threads from other car forums:
Let us know if you have any luck taking this to the next step.