I have a set of 18x9 wheels for my mustang. Can I fit a 235/50/18 tire? I plan on a better set in the future, but I found a deal on a set in this size to use right now.

 

Max. rim width for a 235/50/18 is 8.5" (according to Toyo's site).
Also, let's not forget 9" = 228.6mm. That is just over 6mm difference between rim and tire -- talk about a stretched-out look. Plus, I doubt the sidewall would hold up well.

Possible? Probably. Not worth it though IMHO. If they pop on the fwy, it won't seem like a good deal afterall.