Almost all places (except for Oklahoma, from what I understand) consider a 275/60/15 radial or a 28x10.5 non-W small tire. Some of the daily driver type classes will allow a larger metric tire, only if they're on a 17" or larger rim. My car has a 295/65/15 and has to race in big tire everywhere because it's 20mm too wide.

Radials require a different suspension setup, they like to be planted with separation and need to dead hook, and need tighter compression settings.