First you want to prep an event.

My suggestion would be:
It gives you 1870 Credits if you place first. (Unsure if best lap adds, because spin-out's and shunts and the like don't.)

Big Valley Outer Oval is fast, roughly 28 seconds for a clean lap, and it's more open so you'll have an easier time getting through the field. (I recommend staying close to the white inner line in the corners.)

Car choice doesn't matter, pick what you like, as long as it's A class. (The same setup with a C class will give you about 1300.)

6 laps because fewer means less reward and more doesn't really increase it, as well as take more time, lowering Credits/time. (Haven't explored this too thoroughly, only testing 1,3 and 6, at which point I thought that each race was getting slightly leinghty, and I had enough time to get stuck in traffic or catch someone with similar performance level.)

Grid size should be maximum, because every opponent adds some reward.

AI level should be the same as yours or more, otherwise the reward system will shut you down.

Difficulty multiplier is at 1.25x, but I am unsure if assists dock pay and not only XP, but I think AI difficulty will give you more if it's harder.