The STR class is a few years old now.
The local SCCA chaptor (and Equipe Rapide club) has supported a Tire class (for street tires) for many years. The RT class on a National level only supports vehicles that could be included in a Stock class. If you have modified your car for STR, then it is not eligible for RT. You would run in STR.
The other three clubs (NTAXS, PCA, and BMW) all have classes that support street tires.
Go to www.dfwautocross.com for a schedule of all the local autcross events and Like us on Facebook at facebook.com/dfwautocross.