here is the guide. works for me and maybe for you too.
notice that you have to change the settings in your .plr (GTR2/UserData/your driver name), not ingame.
i dont know if you can change setting when the game is running. it´s a bit annoying but i close the game, modify, start and try and again...
take your time and do 3-4 laps after every change and choose a track you know well (Hockenheim, Barcelona are good test tracks generally)
Dont forget to save your .plr before for safety reason.
I recommend to take a low downforce car (no open wheeler or similar) with a setup you are used to.
The Porsche 997 cup for example.
Additional set all update tresh values to 0,00000
feel free if you have any question.