GTO are mid 90s muscle cars that can run on both ovals and road courses, although in real life, they never ran on ovals. They were part of the old Trans-Am Series, that was mostly in the USA. Some say they remind them of an old school stock car with better handling, which is why they are great for ovals. Group C1 are the early 90s Prototypes in GT style that were part of the Wold Sportscar Championship. These cars were almost as fast as F1 cars, with a higher top speed. Perfect for ovals and road courses, since they love to go fast and sound great, and strike a balance between GTO and GT1, but are faster than either. GT1 2000+ FIA spec cars that preceded the current GTE Class. They were not designed for ovals, so they are not really a challenge to drive on ovals, but are not just a walk in the park either, because of their speed. On road courses, they fly.