PC1 How to modify the 'tunable parts' on a car?

[Brave Sir Robin]

Hi all,

I would like to know what i have to do (what files to edit and how) to add some 'tunable parts' to Road Cars, for example an adjustable Final Gear in cars that don't offer that, like an Audi R8 V10 or a RUF RGT-8.

Since there are mods for this game, i am pretty sure there is a way to do that but i have no idea how. Can anyone help?

Thanks in advance,
Sir Robin