What I do when I want a custom race with my tuned road cars:
- each car is equipped with all the available Racing tire (Hard to Heavy Wets). That means you have to spend a lot for each car. And multiply that to 20 for a full grid.
- AI will use the RH tire as default, then will switch to Heavy Wets when it rains. If it stops, they will switch to RS tires and do the usual rubberbanding which makes them record a lap much faster than me.
- Alternatively, you can only equip the car with the RH tire and the AI will still use other racing tire compounds during the race once they pit in.