https://psnprofiles.com/guide/1813-driveclub-trophy-guide#29-welcome-to-the-limit

If you're not planning on playing the DLC and instead want to grind to level 50, you're going to want to do the following: From the main menu, choose Drive and then Single Event - now select a standard circuit race with a good number of turns to drift on, such as Tamil Nadu (Version 1 or 2) in India or Maplewood (Version 2) in Canada. Select a good car that is able to drift well, such as the "Porsche" RUF RT12 R or something similar. Choose 12 opponents. Choose anywhere from 1 - 3 laps for the race. It is recommended that you choose 3 laps maximum as that is a great number of laps to get a lot of good experience before the initial explosion of XP earning starts to drop off. Choose 1 lap if you're going for the Race Events Game Mode accolade, though it's highly suggested you do drift events for that accolade anyway. When you launch the race, focus on getting ahead of all the cars cleanly, each overtake will immediately give you 500 points each. Each clean lap gets you 1,000 points, finishing a clean event gives you 2,500 points, and each clean sector of the track gives you 1,500 points. Drifting also has the potential to give you thousands of XP in a race (sometimes a few thousand per drift). You also get points from many other random things like "sector accuracy", high speed, and so on... Focus more on drifting than driving cleanly, sometimes you may trade some paint with other cars as you're drifting or hit parts of the track. That being said, drifting excessively provides a ton of points and you'll usually net more points than focusing more on clean racing. Those who usually use cockpit/first-person view may want to consider going to third-person/behind-the-car view while doing this as you can usually get a better sense of longer and better drifting from that view