ORIGINAL: Riptide
The stock tire on the 18" package'd R/T is a 235/55/R18?
Yes, I believe so. I got it from the Tire Rack website which was correct about the 20" tire, so I assume they are correct about the 18. Their site is pretty cool. Lots of info, and if you are shopping for wheels, they have a link where you can see each wheel on a Challenger picture, and you can even change colors for the car. I don't like any of their wheels, but still a cool feature.
-Dave