https://www.youtube.com/watch?v=526dxk9nnGs

OK so you can either watch the video or read this:

RaceNet challenges for each discipline appear every week, with different track and car combinations. Consider this example:

You really like Endurance racing but can't find anyone to play online with, and when you do, it's always a lobby of two or three people, and you want to level up your Endurance rank and/or a particular car in that discipline. Then, you're SOL. Not only do you have to wait for people to join, but you have to win the race, wait for the race to start, and also wait for the particular car that you want to use to come up. If you go to RaceNet challenges, you know there will ALWAYS be an Endurance event that you can keep replaying. Even if it's not the car category that you're after, you can buy the car (used if you don't care for it), do the event lots of times and get the XP bonus for your car level, and sell it on for a profit! The difference between buy and sell price in this game isn't very much, so it's a worthwhile investment if you have the garage space. The advantage of doing this is that you can restart and can guarantee a win every time and don't have to wait after each race for other people to finish or care about how many poeple are in the lobby or wait for the car/track combination to come up.

You can maybe even rotate each week if you wanted to. So, for instance, let's suppose you want to go for the "I've stopped counting" achievement (get to level 250 overall in online). Then, each week, you can just buy the cars that are applicable for each discipline and do those events. At the end of the RaceNet week, you can sell those cars on (at a profit) and buy a new set.

Alternatively, you can obviously just set up a custom cup with your preferences and play it that way. But f you wanted to "boost" XP with friends, you'd have to take it in turns to win; which is much less efficient than having guaranteed wins against AI.

Note that this guide is not intended to tell you how you should play online. Of course, it's much more fun to enjoy the variety and the unpredictability of real online racing. This guide is just intended for those looking for a quick way to get certain acheivements or upgrade their cars, and in no way do I think anyone should spend their entire time on this game replaying RaceNet challenges.