Forza Motorsport

You earn CarXP while racing. Doing good in a segment by following the racing line and making good or perfect turns (as they were in prior FM) give you a score (0-10), the higher the score the more XP. You get bonus XP for scores over 8. Additionally you gain XP for passing, so setting yourself in last place helps in career.

You will also be required in Career to do a practice race (optional in multi-player). During this race you will have sections where you need to set a time. Each time you set a personal best (including the first time) you gain XP. A strategy would be to go slow your first lap and gradually increase your speed each additional. You also get bonus XP for beating the practice time based on your difficulty level (career only). These sections also exist in Rivals if you are trying to knock out those achievements.

For reference I played at difficulty 2 and to be honest not the best at Forza or the segments (averaged a score of 5-6), and by the end of the first Series after the tutorial my car was at level 22. Second series I was better (averaged 6-7) and hit 25 almost 26 in the last race. So it's possible to get this from each series if you race well, if not you will still need to grind to 50 anyways.

After using 20 different cars in career (you will be doing this to complete 4 tours or 20 series), you can play the series again with the same car, drive it in multiplayer, or to grind out the 30 cars needed at 50, go to Free Play, choose any track with a few easy curves (nothing too tight) or oval track (you will get less segments so less XP) and however many laps or time you want to commit. Then in assists turn on everything to fully-assisted including acceleration. When the race starts push the to start going and then the car will drive itself.

This video includes how to earn infinite money fast in Forza Motorsport. And how to earn infinite XP fast in Forza Motorsport.