To add to the other solution(s): Leveling PI Advice.
I found that after a while I'd rather just auto-race, especially when looking to level up the PI of my cars. To do this I needed to maximize my challenge packs, IRL time, and avoid spending wrenches, which meant that I needed to be able to auto-race every race in a given event.
Once I played through the first episode, I began doing the daily upgrade and special races as well. After getting a sports (yellow), street(green), and super (blue) car to 514 or so, you can start grinding easier. You'll need to be at least Level 15 to have 3 cars to race with.
Episode 1, Street Event 6, gives you 7 races if your follow the lower path. By auto-racing and choosing the +300 match ups, I was able to get 4 challenge cards a run in about 4-5min IRL. You can do this twice at level 15, based on the amount of energy you have at full, before having to wait or get partially through the event for a third time before having to stop and wait.
Doing this for about a week, I was able to level 12 cars to 514, and I picked my best ones to level higher. Why 12? Some of the special races and story races require a Modern Super, or a Classic Muscle, or a Retro Street, etc. 3 types of cars x 4 car "classes" = 12, if you want to be prepared. You'll need the three various types of cars to be leveled in each class to do the "class " specific races and get the upgrade parts you need, as you can only race for "class" upgrades using class cars (except for the wild event upgrades which are random).
Once I got a muscle, super, and sport, to 540-550 PI, you can start auto-racing and getting 5 challenge cards a run.
Episode 1, Muscle Event 4, gives you 9 races. By running auto-race at +300 that's 27000, enough for 5 challenge cards. As before, I went the lower path and it took maybe 4-5 minutes to do the entire event (if I was watching TV or doing something else). I could do it twice without issue before needing to wait for energy or only getting a portion of the way through the event for a third time. You will use a sport, muscle, and super (any type is fine), but if you want to auto-race them all the highest PI needed is 541.
There were a few set backs, in that I would end up getting a better quality car that was a much lower PI than the one I spent time on leveling but I like to think it was probably better in the long term to level up that silver or gold card over a generic one. As other guides have suggested, always level 3*** cars that you need, ignore 1 and 2 stars. if you end up getting a four star car, level that too, even if it is common.
Edit: Save your gold for cars. You'll want to get enough for 5 cards or 10 cards (price is the same rate).