Just set up quick race:
1)for duration of 10min or whatever you prefer (going to need time to pass everyone)
2)set to an AI difficulty that you have and edge (80-100) for the length of race you set
3) Set 10 opponents.
4) Set yourself to start in P11.
5)Then Race passings all opponents by end of race.
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I did this on Paul Ricard. The long straight and a fast car (like Audi, Aston, newer Bentley that can easily do over 280kmh in slip stream ) it’s pretty easy to pass most of opponents in 2 laps. That leaves you About 3 laps on 10 min race to pass the top three.
If you don’t succeed lower AI difficulty and/or up race time