I have a big receiver that takes up some room, and the magnaflow system that the PO put on runs into the area, but the tire fits without touching either. Its close, but not touching.
How close?
If it's only a few inches away I'd be concerned about heat ruining the tire. The exhaust gets *HOT*, even at speed.
I remember seeing some pictures someone posted where they had a basket over the rear bumper (similar to Garretts, but a bit lower). A plastic container they had in the the basket melted, and IIRC the propane tanks got hot enough that they popped the safety valve.
I'd definatly want a heat shield protecting it if it's too close.