if you want to race online casually, iRacing is the ultimate spot (however, it is also by far the most expensive one). You never actually buy the game and / or the additional content, you buy the right to use it for a given time (a month, three months, a year, two years whatever scheme you buy into). Once that time is up, access to "your" iRacing is blocked unless you buy new time.
Once you are in, iRacing offers you a) a racing car simulation, b) a base of sth. like six cars and six tracks (some road / some oval), c) the opportunity to buy access to more cars and tracks (43 cars / 68 tracks in all), d) most importantly: access to servers which run 24hours and offer numerous races simultaneously, bot road and oval.
There are many series to pick from, each run one or more cars on twelve different tracks in twelve weeks (we are currently at the third track / third week for each series). You can choose between playing solo, doing open practice for one of the series with other players, doing a time attack (given number of laps must be done solo as quickly as possible with no mistakes allowed), setting a qualify time for a race or doing a "real" race. In the latter three events, the so called "safety rating" system is applied: every infringement (off track, losing control, hitting another player's car etc.) is automatically recorded and measured against your performance so far. You need to get a good SR (safety rating) to be able to play many of the cars not available in the basic outfit, so everybody keeps an eye on their safety rating most all of the time. This little trick makes for a lot of clean racing
There is a second counter ticking away in the background, your iRating. This measures your strength on the track and is reevaluated every time you participate in a race session. Grids are formed according to this iRating, so you have a fair chance of ending up on a grid with players your own size (which can make for fun, close racing). At the end of the race, your result is measured / compared to all other players on this grid and you are either awarded or deducted iRating points, much as the ranking system works in professional tennis.
Also, you score championship points in each race event, the higher the cumulative iRating of the field, the more points scored. These points add up to a championship at the end of the season (and with all players divided into 12 divisions according to their past performance, even a mediocre player like me has won a couple of these already in the lower divisions).
So: it is VERY expensive, but the iRating and safety rating system together with a solid number of players make sure that you very often end up in a full grid with chaps your size who mostly play fair.
As an offline game it does not work, since there is no AI at all, just solo training. Online is where it's at.