I want to get a set of 18" wheels but not sure what size tires to get. What's the widest size tire I can get without it rubbing or making any mods? Also, Is there an optimal tire size for 18" wheels on the 09 xB?

Thanks for your suggestions.

 

I would stay around 8.5 +30 area you'll be fine at that width.

 

Tire size all depends on wheel width, offset, and what drop you are on (springs, coilovers or air bags).