I tend to lower the steering ratio because I like to have a more responsive GT3 when I turn my wheel while putting very little angle.
Don't hesitate to raise the steering ratio by 1-2 clicks if you are used to playing with more steering ratio.

TC: - I recommend the TC6 to be safe.
      - TC5 it's a good compromise too but a little bit oversteer to control

HotLap/Qualifying lap adjustement:
- Front Target psi tyres at the end of the lap 26,9-27,0 
- Rear Target psi tyres at the end of the lap 26,9-27,0
- Front Toe: increase 1-2 clicks if you want more stable but understeer appears (it depends on the feeling of player)
- Rear Toe: decrease 1-4 clicks if you want more turning ir don't touch
- Front Antiroll Bar: increase 1 click if you want more stable OR don't touch
- Front/Rear Bumpstop Range: for me this setting is a good compromise. But if you want more stable and more safe, I recommend to put max at the front and at the rear
- Rear Ride Height: increase 1 click if you want more turning OR decrease 1 click if you want more stable OR don't touch

Race adjustement (advice): Adjust based on race duration. For 15 minutes, you can keep the qualifying setup. For endurance with a 1h/1h10 stint, opt for a slower setting for pure performance but consistent over the duration of the stint.
- Front Target psi tyres at the end of the lap 27,0-27,1
- Rear Target psi tyres at the end of the lap 27,0-27,1
- Front Toe: increase 1-10 clicks if you want more stable but understeer appears (it depends on the feeling of player). Anyway, in racing, you have to think about tire wear. 
- Rear Toe: increase 6 clicks if you want more stable OR near to 0 OR don't touch
- Front Antiroll Bar: increase 1 click if you want more stable OR don't touch
- Front/Rear Bumpstop Range: for me this setting is a good compromise. But if you want more stable and more safe, I recommend to put max at the front and at the rear
- Rear Ride Height: increase 1 click if you want more turning OR decrease 1-2 clicks if you want more stable OR don't touch
- Front Brake Ducts: it depends on the temperature
- Rear Brake Ducts: it depends on the temperature

My best lap with this setup: 1:42.482 (on a mega lap, there is the possibility of doing 42.4 very low, so you can improve so much my time)

Good Luck and Have Fun.

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