It's not damage. For instance;
Sebring Full pays out at 3200 Credits per lap. It runs 3.7 miles per lap. If I run a 5 lap race, the purse will be 5 x 3200; the difficulty bonus will be (5 x 3200) x Difficulty Setting; and when it is done, I will have driven 18.5 miles.
So that is 16000 credits and XP points for the race. The Bonus, according to the difficulty setting in that menu on my main screen, is 60%. Accordingly, I should have been paid 16000 + 9600, or 25600 Credits.
Now, I ran the race and incurred no damage, yet I was paid only 23200 Credits and I gathered 16000 XP points. A minor bit of arithmetic will show that the 7200 bonus is the result of Difficulty setting of 45%.
I have checked every setting I can find on the setup pages for the game, AI is set to hard. Damage is set to simulation and always on. If I check the Assists menu on the Private Game Main, the settings are as I have them back on the Main Menu.
For some reason, I am getting docked and can't for the life of me figure out why or how. What happens when you guys run these races? Are you getting paid the bonus that is displayed in Difficulty screen of the Main Menu?

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