The RS class of races is the one that gives you more points and it's supposed to be the hardest one (with faster and more agressive AI). This is the obvious part about them.
What isn't so obvious is that in RS races you have to keep an eye on the calendar, because there is a fixed schedule covering one game year and - to enter a RS race in a given week - you have to select the apropriate car (one week in advance). As an example, there's a month (four weeks, four races) for FF cars, one for race cars, one for 4WD cars, one for rally cars, one for race cars, etc. If the cars you have selected don't match that week's RS race requirements, you're stuck to race with them in the RI class (or take the opportunity to change the car if, considering the RS race calendar for the coming weeks, that's what you want to do)
This also means that, while you go up the ranks, you should keep changing cars because you'll want to have - at least - fully tuned FF, RWD and 4WD cars, when you reach the RS races.
btw, the game doesn't consider itself "beaten" if you get to 1st rank. And you also don't even have to win all RS races to do it. What makes you "champ" is the victory in the last RS race (4th week of December), the "King of the Year Race". And that's a race you can't enter with a race car. The S13 Silvia is a good option for it.
Then you can listen to it whenever you want from the library.
